Mal moved his body using his hips and legs in a way that forced his member deep into Terry, his breath hot on her back as he hunched over her and supported himself on the bed. Usually not self-conscious during sex, anymore, Mal noticed strange sensations all over his body growing more insistent. Thinking it was just sweat, he pressed on for a few more thrusts. No, the sweat was really itchy. He changed position slightly, standing upright and holding Terry's hips as she rocked in front of him.
Mal reached backwards for a towel he had left hanging over the closet door. Grabbing it, he draped it over his shoulder... but it wasn't picking up sweat... and his tailbone was starting to hurt. That slight ache spread up his spine quickly, and down through his body. He had to keep going for Terry, but what was going on? Maybe he should stop... he couldn't help but hunch back over Terry again, his breathing becoming ragged. He couldn't make a sound, his throat was so tight...
---
Terry still had her eyes closed and despite Mal doing something weird back there, he was back to where she liked him - deep. His breath climbed up her spine to her neck, where it made her extremely sensitive, and somehow he got deeper. Mal coughed, and Terry noticed that it came from behind her head. Was he on the bed with her? ...what was? She rocked a little slower, but deeper... was he thicker? That was starting to hurt... Mal coughed behind her again, and his hands did something on the bed next to her... HE was REALLY BIG... "ouch!" Terry opened her eyes and looked to her left.
"Holy Shit!" That was not Mal's hand! It wasn't his hand on the right, either! Terry swiveled her feet around, trying to touch Mal's legs... and they felt really weird - hairy and thin?
She tried to crawl forward off the penis that was completely filling her, but it grew longer as she did until it finally popped out. That was a major relief! Terry crawled to the head of the bed and turned around. A horse was leaning over Mal's bed, its forelegs bent under at the hock... huge cock glistening. She looked up, the horse's face was obscured in darkness, but clearly not anatomically correct - and the beast was wildly throwing its head. She reached over and turned on a bedside lamp.
She knew what had happened - the terrible truth caused her to cover her mouth with a hand as she saw Mal's eyes plead with her and then transform into those of a horse. He let out a loud neigh and a snort and was completely equine.