It was the last animal he wanted, but the first one he thought of.
Tom watched with a terrified curiosity as Matt morphed into a bull before his very eyes. He remembered what Kate had said, about living on her uncle's farm. He considered Matt's future there, grazing in a paddock with a couple of cows--
Tom's blood ran cold. Cow. That was the animal he thought of. But was that possible? He turned and looked at Kate with an expression of horror, and immediately she knew.
"It's time, isn't it?" she asked. "You thought of one. Well, here we go. Let's see it!"
Tom fell to the ground as his feet were filled with so much pain, he could no longer tolerate standing. Kicking off his shoes, he saw his toes morph together into a dark substance, hard as obsidian. Meanwhile, his fingers were fusing as well, as his hands shifted and molded into a pair of hooves, identical to those at his feet. Tom timidly stood up on all fours, in his new bovine stature. Kate got the hint.
"Of course, of course," she observed, "you looked at your friend, and you considered his heifer."
Tom panted desperately, "Please don't do this, I'm not a cow, I'm a person!"
"Not for long, I'm afraid," said Kate. "I'm sorry, I didn't mean to do this to you two. I was only trying to get my boyfriend back. But I can't believe it's working!"
And it was working. Tom's swelling body put on about a hundred pounds. His clothes were bursting at the seams. With a final stretch, Tom's jeans were torn to ribbons as the denim gave way to his ballooning posterior, now bare to see. His hips squared up into right angles on either side, flattening out his rear end. At the base of his spine, a tail sprouted with a tuft of hair, snaking its way down over Tom's engorged asshole to its full length, twitching and swaying involuntarily. As the tail brushed his penis, it began to inflate into a round, fleshy udder, which slowly seemed to fill with milk.
From there the changes moved north, as Tom's now barrel-chested physique filled out the rest of his bovine form and approach his neck, which grew a bit longer to accommodate grazing. His head adjusted as well, looking forward, and his ears grew larger and limp, falling to either side of his head. Tom's eyes crossed as he watched his mouth and nose protrude into a bovine muzzle, crying out one last time to Kate, "Why woold yoo doooooooo~", his words slipped into the mellifluous mooing of a heifer.
"Wow, look at you!" Kate squeaked with excitement as she inspected her new livestock. "I just wanted to transform my boyfriend. I got a pair of cattle out of the deal too! Now, what to do with you..." she mused as she patted Tom's rump.