I was very excited when I woke the next morning, and I wasn't disappointed! After the usual morning ablutions, I was astonished, and then delighted, to see Chloe and Scheherazade's groom wheel out a double-seat carriage, clearly designed to be pulled by two ponygirls. I was to be in harness with my fellow ponygirl and occasional sex-partner! I turned to her and saw that she was smiling just as insanely widely as I'm sure I was.

The design of the sulky, although larger, was virtually identical to the single seater. The only real difference was that a single shaft came from the middle of the seat and then branched into three, like the prongs of a Trident. I was backed in between the left and centre prongs and harnessed in place, while Scheherazade was harnessed to my right. We were then led out into the field and began a morning of intense training.

Working as a pair required a whole new skill set. We both had to move at the same pace, regardless of whether we were walking, trotting, cantering or galloping and, as we couldn't look at each other because of the blinkers, we had to develop a kind of sixth sense to understand what the other was doing.

It all became so much harder when we started to practice cornering and turning, rather than just going in a straight line. The ponygirl on the inside of the turn had to go so much slower than the other, who was frantically scurrying to get around the bend in time.

Hour after hour we practised this, starting at a walk and working our way up to top speed. I was grateful when I saw that we were returning to the stables, and I was looking forward to a drink and a rest. I got the drink, as Chloe watered me, but there was no rest for us. Marianne and Conchita walked over from the hotel as we were standing there, waiting.

"We've given them a few hours of rudimentary work as a pair," Chloe explained, "why don't you take them out for a trial?" A few minutes later, we were heading back out into the extensive grounds, initially under the control of Marianne, but then with Conchita taking the reins.

From that moment forward, we were taken out as a pair every day by Conchita and Marianne, rather than the grooms - our training period was over, and for the rest of the time there, we were used by the two women for their pleasure.

It felt different that it was them driving us rather than Chloe or one of the others. The grooms were just doing a job, albeit very professionally - they were hired hands, ultimately employed by us and answerable to us (with the proviso that it wasn't actually me paying the bills). I assumed that they were restricted in what they could make us do by some kind of health and safety regulations.

Conchita and Marianne, however, were something very different. There were no agreed limits about the use of the whip, or how long, hard or fast they could drive us. They were totally in control of us, and we had to obey or suffer the consequences. That was the thing which, somehow, made it so deliciously erotic.

The days began to merge and lose their individual identity - each started early with the toilet routine, each ended late, and in total exhaustion. I knew I was getting fitter and stronger, but it made no difference at the end of the day - we were being worked so hard that we fell into our straw beds and were asleep almost instantly. Scheherazade and I didn't even have enough energy to pleasure each other before we were put away for the night, and were reduced to lying together on the grass, just cuddling.

Wednesday was notable in that our hands and feet were once more treated to make sure there were no long-term issues with wearing the boots and gloves. Then, at lunchtime, we were driven back to the stables, which was very unusual, and brought to a halt near the side door.

A few minutes later, the door opened and out came Scarlett and her partner - although she was no longer dressed as a ponygirl, she was in proper clothes, wearing make-up, with her red hair nicely washed and coiffed. She came over to us and, smiling, stroked our faces.

"Thanks for bringing them back," she said to Marianne and Conchita, who were still sitting in the sulky.

"No problem," Conchita replied from behind me.

"Goodbye, ponies," the woman who had been Scarlett said, now looking at us, "it was fun. Thanks for including me in your little games." She kissed each of us on the forehead and then left, arm in arm with her boyfriend, walking back up to the hotel. I felt the reins pulling, and I turned away as we were driven back out into the country. I was sad to see Scarlett leave, but that was soon forgotten as I concentrated on keeping in step with the girl next to me.

When Sunday came, there was another gaggle of wannabe ponygirls milling around, but they didn't really distract me from the task in hand which was, as always, to obey the commands transmitted through the reins and the whip as quickly and accurately as possible, to satisfy our demanding drivers.

When the second Wednesday dawned, I was so sleepy that I didn't realise it was anything special. But, as Chloe led me out of the stables and round to the side door, it finally sank in - this was it, my last day! I sat there in a bit of a daze as Chloe stripped me of my bridle, harness, tail, gloves and boots, before handing me a towel and leading me to the back of the room, where there was a set of showers. Under one of them, to my surprise, was Scheherazade!

"Hey there, Sarah," she said, smiling and holding out a wet hand, "come and join me. How does it feel to be human again? My name’s Tanya, by the way." I put down my towel and stepped into the shower alongside her, enjoying it when she pressed her warm, wet body against me. Suddenly, our hands were all over each other and we were kissing for all we were worth. Long story short, we very quickly brought each other to a very noisy orgasm!

Once we had got that out of the way and recovered, we washed each other, luxuriating in the warm water and soapy suds. Eventually, we clambered out and dried ourselves, before walking back into the tack room, where we found the boxes with our names on and clothes inside. I noticed that Tanya's full name was Tanya Davies.

It felt weird to be wearing bra and panties for the first time in two weeks, and Tanya and I, appropriately enough, horsed around for ages, giggling together, looking at ourselves in the mirror and pretending we were still ponies, strutting and prancing around.

When we were finally dressed, Chloe told us that Marianne and Conchita were waiting for us at the hotel, and that we should hurry up because they were waiting to leave. I wondered what the rush was, but we hustled over there and found them waiting outside by a car.

"Come on, you two," Marianne said, "we've got a plane to catch!"

"What?!" I exclaimed, "where are we going?"

"Get in the car," she replied urgently, "we'll explain on the way."

As when we had arrived, we had to wear blacked-out sunglasses to prevent us from knowing the exact location of the stables, so it was rather odd to hear Conchita's voice so disembodied.

"I suggested to Marianne that we could all spend some time at my father's villa near Mexico City - cocktails around the pool, lazing in the sun. It is also my dad’s fiftieth birthday and he’s having a massive party, which should be fun - what do you think?"

"Sounds great to me," I said enthusiastically, "I could do with some R&R after the last couple of weeks! But I don't have my passport or any holiday clothes."

"Don't worry, Sarah, I got your mum and dad to send it to me, and I went shopping for you. What about you, Tanya?" Conchita asked.

"If Marianne says that’s what we ‘re doing," I heard Tanya say, "then it's decided. It sounds lovely! Thank you for the offer, Miss." This was such a strange answer that it made me remember how Marianne had described herself as ‘Scheherazade's owner’. Was that just a ponygirl thing, or did it extend beyond her time at the stables? I wondered again what the relationship was between these two.

The car took us straight to the airport and, after we had unloaded our luggage, we went over to the check-in.

"Good morning, ladies," the woman said, "I'm authorised to offer you a free upgrade today, but unfortunately, we only have two spaces in business class left."

"No problem," Marianne said confidently, "just put those in the names of Duchamps and Alvares, these two will be happy with economy." I opened my mouth to protest at her assumption and downright arrogance - I had never travelled in luxury at the front of the plane, and I quite fancied trying it - but something stopped me. Tanya was completely accepting of the decision, and I could see that Conchita was happy with it too. She was looking at me hopefully, maybe expectantly, clearly wanting me to go along with the idea. I closed my mouth and followed on as the others left the check-in counter.

Once through security, Marianne and Conchita headed off for the club class lounge, while Tanya and I had to make do with a drink bought from one of the shops with the little cash we had - neither of us had taken much with us when we went to the stables. We ended up sitting in the overcrowded terminal with nothing to do and not really wanting to talk together in case we were overheard. At last, we made our way down to the departure gate.

On the plane, luckily there was nobody sitting next to us, so we could chat away happily. I told her all about me and Conchita, and how she had all but tricked me into going to the stables and subjecting myself to the ponygirl training.

"Oh come on," Tanya said, "you loved every minute of it! Don't you deny it!"

"Okay," I replied, mock-reluctantly, "I can't deny that I did get some enjoyment out of it, but still! She shouldn't have tricked me! I would never have gone so extreme if I'd known!"

"And you wouldn't have got half as much out of it!" Tanya countered, "the whole thing about it is the total immersion - you can't go about it half-hearted! By the way, I have a little secret to tell you. You know those high-tech collars they made us wear? Complete bullshit!"

"What do you mean?" I asked in confusion, "when I tried to talk, the collar gave me a shock!"

"Oh yes, they’re shock collars," Tanya said conspiratorially, "but all that stuff about them detecting whether you're speaking or not - that's rubbish. The trainers each have a remote control and, if they see or hear you trying to talk, they press that! The technology to monitor your vocal chords and distinguish between a word and a squeal just doesn't exist!"

"So why didn't you talk to me when we were away from them?" I asked accusingly.

"That’s simple - ponies don't talk! You’ve really got to live the part, not just obey the rules because they make you!" Somehow, I understood what she meant. Nobody had forced us to go there and become ponygirls, so why should they have to force us to behave like ponygirls? It made me think rather more deeply about my recent experience, and we fell into silence for a while.

"So," I said eventually, "you and Marianne -what's that all about? Tell me the whole story!"

"Well, I went to work for a company owned by Marianne," Tanya began, "and, for a few months, everything went along normally. I've always known I was gay, and never tried to hide it. Most people don't have a problem with it, and I don't shit on my own doorstep - by which I mean, I never tried to pull anyone at work, and certainly not Marianne - she was way too important and, I thought, straight as a die. So imagine my surprise when she made the first move!"

"She asked you out on a date?" I interjected, more to express my interest in the story than to ask a question.

"Not exactly," Tanya went on, "she asked me round to her house, which spooked me a bit - I was just a junior assistant nobody and she was the boss, what could she want with me? Anyway, she sat me down and, quite calmly and coolly, explained to me that she was a Dominant, and she wanted me as her submissive!"

"Oh my God!" I squealed, making several people turn around and look. "What did you do?" I whispered.

"I asked her what she meant," she replied, "I had no idea about dominants and submissives, it was all new to me. So she explained, still in her cool and calm voice, that she wanted to strip me naked, tie me up, and make love to me!"

"Oh my God!" I squealed again, causing the same people to turn around and give me a filthy look. I apologised and turned back to Tanya, who was sniggering into her pillow. "What did you say then?!"

"You’ve seen Marianne, she's a very hot chick," she explained patiently, "what do you think I said? I said ‘yes, please’! She told me to stand up, then pulled my top off over my head and then unfastened my trousers, telling me to take them off. She then ordered me to strip out of my underwear so that I was standing naked in front of her, while she was fully clothed, looking me up and down and smiling, like she liked what she saw - that felt like the most erotic moment of my life so far! She told me to follow her, and she led me upstairs to her bedroom, where I had to stand to attention while she got out her ropes and tossed them on the bed.

"She made me lie on the bed and then tied me down, splayed out like a starfish. Then she said she was going to heighten my sensitivity by removing my other senses. I had no idea what that meant, but I let her put plugs in my ears and then a leather hood over my head, which had no eye holes. I was getting a bit nervous and a bit scared at this point, but there was nothing I could do. I just kept quiet and let her get on with it, deciding that, if it was awful or she beat me or something, I was just going to have to put it down to experience - a really bad experience, like the worst date ever.

"Anyway, to cut to the chase, she gave me more orgasms in one night than I had had in my entire life! She used her tongue, her fingers, her toys - she seemed to have every sex toy in existence! By the next morning, I was hers completely- I was happy to do anything for her, on the off-chance that she might, one day, give me another night like that. And I can tell you - she has given me many such nights!"

"So you became… what exactly?" I asked, still wondering about the relationship.

"Whatever you want to call it," she replied with a happy smile, "lovers, life partners, Domme and sub, Mistress and slave - all of those are appropriate at one point or another."

"Wow!" I breathed, overwhelmed by the depth of the link she was describing, "and when did you first go to the stables?"

"That was a couple of years ago. I never had ponygirl fantasies like you, so it came as a complete surprise when Marianne booked it without telling me, and we just turned up there. We've been back every year since, so that was my third time and, I have to say, you made it the best!" We exchanged a warm kiss, but then a troubling thought occurred to me.

"I'm sorry if our ‘messing about’ causes you two any problems," I said worriedly.

"Don't be silly! " she laughed, "Marianne knew all about it from the grooms as soon as it started, and loved the idea that we were having fun. As far as she is concerned, it seems a perfectly natural thing for ponygirls to do."

Our conversation continued, ranging across many topics, and we were firm friends by the time the plane landed.

I was very impressed when we were taken from the airport to the villa in Conchita's dad's private limousine by his driver. The property itself was on a hill overlooking the sprawl of Mexico City, and the gentle breezes provided sweet air and welcome relief from the oppressive summer heat.

The villa was set in magnificently manicured grounds, and the drive up to the main building was long and shaded by trees. As we approached the front entrance, a tall, distinguished-looking man with greying hair at the temples was waiting at the top of a flight of white steps to greet us.

"Ola, Papa!" Conchita exclaimed as we got out, running up the steps and throwing herself around him. They hugged for quite some time, before she turned to us, smiling broadly and keeping her arm around him possessively.

"Papa, these are my friends," she said proudly, "Marianne, Tanya and, of course, Sarah! Girls, this is my father, Jorge" He came down the steps and shook us all by the hand in turn, until he came to me.

"Ah, Sarah," he said warmly, "you I have heard so much about in Chi-Chi’s letters from school. All your adventures together! It is good to finally meet you." He enveloped me in a hug for almost as long as the one he had given his daughter, which was nice, but also a little embarrassing and unsettling. I wondered what exactly she had told him about me.

We went inside, having been told to leave the driver to sort out the cases, and we were served champagne, or the Mexican fizzy equivalent, by the maid, introduced as Maria. The villa was enormous, high ceilings, tiled floors, white walls, and those huge, slow-turning fan things on the ceiling rather than air conditioning. It felt wonderfully cool after the broiling heat outside.

We were taken out onto a veranda, which led to an Olympic-sized swimming pool, and it was clear that Jorge was a wealthy man, whether his money was ill-gotten or not. We sat down at a table and Marianne described her business to him, while Maria provided plates of hors d'oeuvres and topped up our drinks. It was so long since I had had an alcoholic drink, I found it quickly going to my head and, with the jetlag setting in, I began to nod.

"Pardon my manners," Jorge said, "you ladies must be very tired after your long journey. Perhaps you would care for a lie down before dinner?"

"I've got a better idea," Conchita responded, "why don't we all go for a dip in the pool?" Everyone was enthusiastic, and Conchita led us to our rooms.

"This is yours, Marianne, Tanya," she said, showing them into a large room with a king-sized bed - I was happy to see that they were being accepted as a couple here.

"This is your room, Sarah," opening the door on another beautiful space, "I've got your bag in my room - I want the clothes I bought you to be a surprise. I'll go and get your bikini."

"Here you go, honey," she said as she came back into my room with the bikini, "come out when you're dressed." Even as she held it out to me, I could see that it was going to be skimpy. When I took it and held up the scraps of material, I knew it was going to be embarrassingly skimpy.

"Chi! I can't wear this! Come back!" I protested but, with a wicked smile, she left the room and closed the door. I looked down at the almost-bikini and felt unhappy. I had been tricked again! Forced into exposing myself before strangers, just like at the stables! As the memories of that flooded back, I began to feel turned on by the thought. Maybe I needed to be pushed before I would take a risk? Gradually, the idea of having to wear this turned from one of horror and betrayal, to one of excitement and anticipation.

I stripped out of my rather stale clothes and pulled on the bikini bottom. It was, as I had suspected, virtually a thong, exposing my buttocks completely, and with only just enough material at the front to cover my shaved pussy - good job I didn't have to wax my bikini line!

The bikini top was just a band which went around me, fastening at the back and with a large shiny metal ring in the middle at the front, joining the two pieces of material barely covering my nipples, let alone the swell of my breasts. A string from the ring went around my neck to provide a minimum level of support and insurance against the whole thing just falling off.

I looked at myself in the mirror, and almost felt more exposed than if I had been naked. However, the powder blue material suited my complexion well and, apart from the completely inappropriate style, Conchita had chosen well for me. I looked hot, like some Las Vegas stripper! Taking a deep breath, I went out into the hall.

The other three were waiting for me. Tanya was wearing a classic white bikini, which just looked amazing against her dark skin. Marianne was wearing an elegant, black one-piece swimsuit, with a silk wrap tied around her waist, black sunglasses and a straw hat shading her face. Conchita was wearing a floral sundress - possibly with a bikini underneath - which seemed to me to be rather unfair, given what she had made me wear!

"You look so sexy!" Tanya said enthusiastically.

"Right back at you, kid!" I replied, "although I think I look a bit more slutty than you."

"Nothing wrong with slutty, if you’ve got the right audience!" She glanced over at Conchita, implying that she was the target audience. Really, I wondered, do I want to look slutty for her?

We went out onto the veranda and Conchita's father was there in a rather revealing speedo. I wondered if this was the fashion in Mexico, or whether he was just a lecherous old goat. He complimented us all on our appearance, but his eyes lingered on me, in what I took to be rather a rude stare, and a smile crept onto his face.

"That's a very interesting pattern of tan lines you've got there, Sarah," he said, making me look down at myself. Holy crap! I’d seen the marks, of course I had, in the mirror just now, but it had not registered, because I had been looking at the same pattern for the last two weeks, except that then it was the dark brown of the leather! Now I was hatched with white lines as if I was still wearing the pony harness!

"Oh! I-I… I, er…" I stammered, turning the deepest crimson in embarrassment. I couldn't think of any explanation other than the truth and, when I looked at my friends, nobody was coming up with anything brilliant either.

"Don't worry about it, Sarah," Jorge said, his smile broadening, "you young people get up to all sorts of things, and I'm sure I don't need to know the details!" My mortification was finally relieved when Conchita pulled off her sundress and grabbed my hand.

"Come on," she cried, "last one in the water is a loser!" She ran towards the pool, pulling me along with her, and jumped in with a squeal. Moments later, we were all in there, splashing around and laughing. The moment passed, and we spent the afternoon sunbathing, swimming, talking and drinking very fruity, but thankfully non-alcoholic, cocktails.

When, eventually, we had had as much sun as we could stand, we retreated to the cool interior, spending a couple of hours before dinner sleeping off the jetlag.

"Hey, sleepyhead," Conchita said later, poking her head around the bedroom door, "take a shower and I’ll lay out what I want you to wear tonight on the bed."

"You're the boss!" I responded, smiling at how she seemed to be enjoying the sense of power controlling my wardrobe had given her. I didn't object, it was actually quite fun.

When I returned from the ensuite bathroom after a long, luxurious shower, I saw what she had left for me and felt the heat rise in my cheeks, and also in lower regions - much lower. I picked up the underwear provided, which consisted of just a red G string thong, with the front piece shaped like a heart. I put it on and moved on to the footwear, red open-toed sandals with a one inch platform sole and four inch spiked stiletto heels. Previously, I would have struggled wearing these but, after my experience at the stables, they were a cinch. I looked at myself in the mirror, admiring the way the high heels shaped and firmed my calves and buttocks. Damn, I looked hot!

And finally, the dress. It was red and, obviously, it was backless, halterneck, low-cut, and short - very short. As I checked myself out, it reminded me of the dress I had been made to wear to the ball at school, except that this was the short version - it seemed like my dress-buying friend liked me in red. The bedroom door opened.

"My God!" Conchita smiled, looking me up and down, "you look even sexier in that dress than I had imagined when I bought it!"

"Do you want your dad to think I'm a hooker?!" I protested, hands on hips and pouting.

"Don't worry," she replied, "he loves looking at beautiful girls! I want to make this birthday very special for him."

"You don't expect me to fuck him as well, do you?!" I asked, only half-joking.

"Don't be silly!" she laughed, "I don't procure women for him, and I certainly don't intend to give you away! Come on, let's go eat." With that rather cryptic comment, she turned away and I scurried to catch her up.

The next few days were wonderfully relaxing, with most of the time spent just lazing by the pool. I was getting used to Conchita's taste in clothes, and rather enjoyed being her dressing-up doll, even when she made me wear a black latex swimsuit - it could have been so embarrassing, given that it showed every lump and bump underneath, particularly my nipples, which seemed to be almost constantly erect, what with the cold water and the wildly erotic feel of the rubber. But I think I was beyond embarrassment at that point.

At last, Saturday arrived, the day of the big birthday bash. Conchita said she had something special for me to wear - no big surprise there, I thought, it's bound to be something slutty and revealing. But I was not prepared for what came next…