Worry and fear managed to break through the dull contentment in Mike's mind. This was all supposed to be just a game, he didn't want to be a dumb animal for real. He should have never gotten himself into this, he just wanted to go home and be a normal human boy again.
Ty frowned. [It's not that simple. The spell you're under is very powerful. So powerful, in fact, that as far as my magic is concerned you really are a plow horse from this world and not a human boy from ours. I can't send you back home, and there's only so much I can do for you here. You'll still be stuck on this farm, for one thing. And I can give you a human form, mostly, but your essence will still be a horse's.]
Mike struggled through the dull confusion to try to fully grasp what Ty was saying. Form, essence, what was the difference? He would take whatever he could get.
[Alright, I'll see what I can do. You know, this might actually be fun. Or interesting, at the very least. I'll try to come back in a year or two to see what else I can do.] Ty grinned, and then vanished in a blinding flash of light that engulfed Mike's whole world and left him disoriented.
Finally, the light died down and Mike regained his footing... on two feet. He was human again! Except things still felt wrong. His head was still feeling thick and foggy, so it took him a while to realize that the human body he was staring down at wasn't his own, and even longer to understand what was different. He was bigger in every way--taller, broader, thicker. Broad shoulders and a body covered in thick, bulky, powerful muscles that he had never had before. He tried to think just how tall he was or how much he must weigh now, but thinking of the numbers just confused him. And most surprising of all was something else that was bigger--between Mike's legs was swinging the huge black cock and balls of the stallion that he had been! So that's what Ty meant by a "mostly" human form.
Mike could see all this, he realized, because he was completely naked. He could also see that his big new body was caked in a layer of grime and dirt... except for where he was being cleaned, because the boy was still grooming him, apparently oblivious to his change. And the grooming still felt just like it had when he was a horse, pleasant and relaxing, so he couldn't help but lull him back into a contented stupor, out of which he was only awoken (he didn't know how much later) by the boy's voice.
"Feeling good, boy?" the boy asked. "Is there anything else you need, or are you ready to work?"
Thinking of words was hard. "Uh, yeah. No. Good, ready," Mike said dumbly, thickly, in a voice that was deeper and slower than it seemed like it should be.
The boy grinned. "Fantastic. You're going to be great at this, I'm so glad we got you. Just follow me out to the plow and I'll get you started." The boy held out his hand, and Mike, still feeling the connection from before and the loyalty and trust that it brought, took it and followed, obediently letting the boy lead him out of the stable. Along the way he saw a water trough and felt thirsty.
"Uhh... drink?"
"Go ahead, fella."
Mike lumbered over to the trough and plunged his face straight in, drinking like a horse and only realizing that there might be something wrong with it once he was done. And then he caught his reflection. His face was coarser and uglier, with a sunken forehead and heavy brow. But what he really noticed were his eyes: the liquid brown eyes of a horse.