At the top of the West Tower, Professor McGonnagall's quarters were indeed freezing. The winter winds whistled through the cracks in the stone, and even when Minerva lit the fireplace with a flick of her wand, it did little to deaden the cold.
"I ought to ask Filch to patch the place up. The cold is terrible for my joints," Minerva sighed to herself in her lilting Scottish accent. She rubbed at the arthritic joints of her fingers, as gnarled as the wood of her wand.
She was seated at her desk, marking student papers in the quiet after-lesson hours, and sipping tea to warm herself. She spoke to the only creature in the room, Scabbers, absently as she would a mindless animal, while he knelt, leashed, at her feet. "Och, but I've got a fuzzy hot water bottle to warm my bed now," she said with a wink down at her confiscated pet.
Peter Pettigrew trembled - not from cold, for the body Minerva had given him was amply equipped with fur and soft fat to keep out the cold, but from fear. After twelve years hidden as the Weasley family rat, he'd become accustomed to people talking down to him like an animal. Normally it reassured him that his disguise was holding. But with Professor McGonagall, it terrified him. He knew the fates reserved for rodents in her care. She'd been his own transfiguration teacher long ago, and he could still remember how the shelves of her classroom would fill up with snuffboxes and teacups each school year - the remnants of hundreds of mice transfigured into trinkets (some successfully, others grotesquely unsuccessfully) by her students.
Would his owner - dear, lovely, sweet, kind Ron - even notice if his pet was transfigured into Mcgonnagall's hot water bottle, wedged each night between those wrinkled, old thighs? He whimpered. Did such a horrible fate await him?
"Never you fret, dearie, I'll be ready for you in a second," Mcgonagal crooned, draining the last drop from her cup and setting it down in its saucer. She dabbed ladylike at the corners of her mouth, then folded an animated picture frame of her husband flat down on the desk, before drawing her wand. "Nothing like a cup of tea to get my blood going. Let me just change. You'll have to forgive me, I've not used this spell since my husband passed." she tittered down at the rat girl and tapped her wand against her chest. "Serpens mutatio."
The narrow outline of the old witch began to bulge and bubble beneath her robes. A wave of thick muscle rippled along her limbs and up her neck, which stretched taller on a sinuous neck, her eyes turning yellow and slitted. Her robes ripped around muscular thighs, exposing aged, wrinkled flesh rapidly filling out into a powerful, youthful physique, gleaming with emerald scales. Her wizened lips parted to show sharp, serpent teeth lining a maw wide enough to swallow him whole.
Pure, rodent fear filled Peter. As he turned to run, he slammed into her muscular tail, which had already snaked its way around him, the cold coils closing in around him. Minerva shuddered in pleasure as Scabbers' hot mammalian body pressed against her reptilian flesh.
McGonagall tapped her wand against her crotch. "Priapisssmos."
Pinned within her coils, Peter's eyes went wide as saucers as what seemed like a second python swelled into existence from her once-female crotch, its vein-encrusted length rising high above him, its girth almost as wide as his waist. "I always wanted to ussse that sssspell," Minerva hissed in delight.
"I surrender!" Peter squeaked desperately. "I'm Peter Pettigrew! I betrayed the Potters! Please don't kill me with that thing!"