Donkey, so far as he could tell.
He coughed down the last of it and wiped his lips. There was a bitter taste at the back of his tongue and a little tingling down his throat as it went but once it had, nothing.
"Not really sure what I just drank," Tom said, smacking his lips.
Matt went over to the vials himself and was looking through while Tom backed off. "But you drank it anyway. Better hope it isn't poison."
"Yeah, right" Tom joked. Just then, his stomach grumbled and there was a little wobble in his balance. He spread his feet to stay stable. Shaking his head, be blinked away the stars he was getting in his eyes.
Matt walked back to the door and checked again. "Really sure there wasn't a note or something else?" Matt couldn't see a thing. Sighing, he closed the door and turned around. "I'm not really sure what.... the hell is going on with you?"
Matt was staring at Tom, who was leaning against the wall, his pants looking insanely tight as if they had shrunk suddenly. His ass in particular was sticking out with the seam down the back now stretching to the point of bursting, with Tom's boxers showing underneath under equal distress.
"Feels so hot in here," Tom tried to shrug off, his feet cramping terribly. He was trying to act as normal as could be, like nothing was happening, then his pants exploded open in the back. His bulbous ass bounced out, his cramping feet suddenly growing numb to the point where he couldn't stand anymore. "Help!" he shouted in vain to Matt as he fell to his hands.
"I...I..." Matt could only hold out his hands at his friend who was moaning in pain.
"It hurts... make it stop," Tom begged while the changes continued.
Deep pops in his hips signaled their shifting of posture and for a moment he struggled to stay up on all fours before it became his permanent position. Tom's thighs became more muscled and hairy, his knees cracking while his socks ripped away to reveal his still darkening hooves. Matt was realizing very quickly his friend was growing hind legs and looking very equine.
The nub at the base of his spine, causing him to cry out further as it felt tugged from his body, only seemed to strengthen Matt's assumption on Tom's approaching species. His butthole began to pucker and grow while Tom himself struggled to breath.
"Oh god, I can't look back. Matt, buddy, what-s HAWWWWW-ppening to me?" Tom begged, his own bray causing him to sob as he already knew the answer.