The Overwatch logo spun behind Mercy, sealing shut the door to her lab. The quiet pneumatic hiss of its lock did little to ease the pounding of her heart, or the tightness in her chest.

“Computer!” Her clipped tone woke the machine from its slumber. Screens flickered to life around her, and she swept her gaze across them, one at a time. “Self-Diagnosis.”

It was a simple command, and one that would only take minutes to complete. Still, Mercy was worried she didn’t have that long. No matter how hard she tried, she couldn’t shake the feeling that there was something truly amiss about…

Her hands drifted lower on her tabard, bumping against the growth between her legs. As a doctor, she knew fully well that it was a penis—just as she knew that there was no reason for her to have one.

It twitched, tenting the soft silk of her clothing.

She didn’t have words to convey her displeasure, much less her horror. She was displeased that her lithe figure was marred by this male appendage. She was horrified that she almost liked it.

Despite her reservations, Mercy stroked her delicate fingers along her shaft. It was more sensitive than she’d expected, and a rush of pleasure nearly doubled her over.

Her palm slapped against a table, and the clap of warm flesh on cool metal seemed to echo through the lab. A thin trail of drool trailed from her full lips to the floor, and her cheeks burned with shame.

“I will not give in!” Though her statement was defiant, she punctuated it with a moan. The simple act of speaking shifted her tabard, stroking the sensitive head of her dick with its cloth once more.

Defiance and aggression welled up within her, and she felt a deep ache from just below her new cock. Again, medically, she knew that the swelling, stretching sensation could only come from a pair of testicles. But as a woman… As a woman—

“Damn! Damn, damn, damn!” She slammed her hand against the table again and again, still leaned forward. The thump of her soft palm grew louder with each impact, not that she noticed. All she could fixate on was how heavy her balls were; how they swayed back and forth.

They’d sway just like that if she was fucking.

The thought was an alien one, but tantalizing. Mercy’s mind focused once again, primal lust flooding her. “C-computer. Activate synth-printer. Pattern: Owner lower body.”

The world of 3D printing had come a long way, to the point where it could create near-organic matter. Making a woman’s lower half was all too easy for her computer, and Mercy turned her head to the left, greedily drinking the sight of a perfect replica of her own ass.

She dragged her hand against the table as she turned, hearing it scuff the metal. Something about that seemed off, but she didn’t care. Her nostrils flared, and she drank in the scent of fresh latex.

“Computer. Lubricate vagin… Vag…” Her mind clouded slightly, and she tossed her head to the side, flicking her ponytail over her right shoulder. “Pussy. Lubricate pussy.”

A new smell greeted her nostrils, and she let out a primal whimper of desire. She took a step towards her printer, and another. The quiet click of her heels turned to a loud crack as she stumbled, her shoes breaking beneath her. The woman’s ears twitched, swiveling with each echo of the sound.

It was an annoyance, but not something she wanted to focus on right now. At the moment, all the doctor cared about was her synth-pussy. Her hands went to the copy of her hips, and she imagined how it would feel. Except… Her right hand couldn’t feel much of anything.

Mercy glanced down, and froze. It was a hoof, there could be no mistaking it. Her fingers had somehow darkened and clumped together, forming a black and thick block of enamel.

At that moment, her computer finished its task. ++Diagnosis complete. Angela Ziegler is undergoing arousal-based transformation into horse. Possible remedies: Unknown. Possible causes: Unknown.++

“A… Horse?” Mercy felt numb. She wobbled back, hearing the loud clopping of her footsteps. She dreaded what she would find, but looked towards them nonetheless. Sure enough, she looked like some sort of equine satyr.

Her once-slender legs had bulked up considerably, stretching her tight uniform. As she watched, thin runs and tears parted the black fabric, revealing similarly black fur. The dense hairs covered her calves and thighs, and she yelped in distress.

The motion unbalanced Mercy, and she fell forward onto the replica of her lower body. Her cock slapped against the crease of her ass, and all panic was washed away by a rush of sheer, primal pleasure.

“Nnh… A horse…” Her eyes glazed, and Mercy’s hips trembled, a small thrust forward damning her further. The pops and snaps of tearing cloth heralded the death of her uniform, and a shuddering wave of change bulked her form up further.

It wasn’t all muscle. Her trim belly swelled into the thick barrel of a draught horse, and she slumped against her hips. “Feels good…” She groaned, vision swimming, head swimming. “What kinda horsey?”

The computer didn’t reply, analyzing her once more. That was good, it left her time to think—or time to act. She chuffed her lips, settling on the latter instead.

Her left hand, still human, stroked the base of her cock. It looked so much bigger now, and its mottled flesh sported the distinctive flared head of a horse. Her balls sagged beneath her, aching with need.

A dimwitted smile spread across Mercy’s face, and she pressed her tip against the copy of her pussy. Then, with a faint grunt, she entered herself.

It was a warm, wet tightness unlike anything she’d dreamed of. Mercy’s jaw went slack, and she let out a coarse, wordless grunt. Her once-proud intellect couldn’t possibly hope to summon words that could define this pleasure. It was like her IQ was becoming sperm, collecting in her balls, sagging her equine scrotum with a needy weight.

Her neck ached, and Mercy’s left hand trembled as she reached to pet it. Her fingers were slick with precum and synthetic lube alike, but that didn’t make the touch any less pleasurable.

Mercy’s shoulders broadened, sundering her armor and expanding her throat in turn. “Nghhkk!” She choked, head jerking forwards as it stretched from her body. Her slender neck swelled, like a sapling growing into a thick trunk. Her hair darkened and lost its shine, pulling along her thick neck to leave her with an unkempt mane.

++Diagnosis complete. Angela Ziegler is becoming a Shire horse. British breed, tall and large. Known for farm work. Fit for…++

The computer kept talking, but its words were meaningless sounds. Only one of them really stuck in her growing head, and Mercy’s thickening lips widened in a smile. “Heehee… Larrr-juh.” She liked the sound of that. She liked being large, having a large cock, having a large body.

Her hips pistoned forward again, and she hilted herself in the copy of her ass. It didn’t feel as good as it did before. It was too tight. It was uncomfortable. Her fading mind struggled to come up with a solution, and her mangled vocal cords struggled to summon speech.

“Cuhmpyu-tuhhrr! Fix! M-make pussy mare puhg-putthy!” Her speech slurred more as her tongue swelled within her mouth, growing thick, growing big. It mashed against her delicate teeth, letting her perfectly feel them coarsen to fit a blocky jaw meant to chomp on hay.

Luckily, her computer recognized the command. The artificial pussy beneath her swelled, as did the copy of hips. Mercy slapped her human hand against the replica of her ass, watching as supple cheeks bulged and rippled.

Soon she was looking at a mare’s hindquarters, complete with a dangling tail. Her own backbone lengthened as if to replicate what her eyes saw, and she felt it brush against the backside of her testicles.

She had balls. The thought, if it could be called as much, stuck in her head. Mercy hiccupped, and furrowed her brow. The motion became more pronounced as her head swelled, and her eyes slid to either side of her reshaping skull.

He realized he didn’t care what was bothering him a second later. He couldn’t remember what he was thinking about either. He was a stallion, and was horny. He needed to fuck.

The transition from female to male happened instantaneously within Mercy’s mind. His first fuck soon followed.

His right hoof slammed against the ground, and his left hand swayed. It was too short to reach the floor, human as it was. That would soon change. Mercy watched his unchanged palm bulge. The darkness of another hoof overtook the last of a woman’s skin, and he felt nothing. No shame, no embarrassment, no sorrow. After all, how could he feel any of those when he was balls-deep in a mare?

“Nuhhr… NGHHREEIIGH!” He bellowed, pulling his hips back and driving them forward. He humped once, twice, a third time. His hips were like a machine, one that once started, couldn’t possibly be stopped.

Even now; though, there was still something left of the woman. Even now, a tiny part of Angela Ziegler’s brain wailed in horror and abject shame at what she was doing. Some desperate sense of self-preservation pushed her conscious mind to the forefront of the stallion’s brain.

Her eyes, still human, still blue, watered. She fumbled for words, even as her body fucked faster and faster, as her balls slapped against her phantom mare.

“NUUUH! THHTUPPP! NUHT UH HGH-HGHHHOOARTHEE!” The very act of speaking was a struggle. She had to force lips not meant for words to form consonants and vowels, and somehow coax a thick and clumsy tongue into enunciation. “UHM NGHHH! NGHHOWT DUMM!”

Despite the intensity of her protests, the passion of her pleasure could not be denied. Still denying her fate, she passed the point between restraint and orgasm.

“UH UH UH NUGHT DUGHMMMGH! GHHH! UH! UH! DUHMUHUHUHDUMDUH—”

And HE came.

“GHHKGHEEEIIIGH! NNHHREEIIIGHH!”

Rich, gushing pleasure surged through his massive length, wetly spurting through his fucktoy onto the ground. With each gooey splat of cum on tile, his eyes darkened further, bright blue forever clouding over into a dull brown.

The stallion forgot everything. He forgot a life in medicine. He forgot how to speak. He forgot a battle angel named Mercy.

The stallion was still horny.

The stallion pushed back, pressing his chest against what he honestly believed was a real mare’s ass, and he began to fuck her once more.