Mike managed a smile. "You think I don't know what you are doing, right?" he snorted and interrupted Aaron's fumbled protest. "You are turning me into a Clydesdale colt, an animal with an animal mind, right?"
An initial intent to deny the truth was pushed aside as Aaron decided this adventurer just wanted the truth. "Well, yes. It will still be you, but you as a horse. I don't think you will remember being human, and even if you do it will be hard to understand. But I was telling the truth about centaurs being in great danger on any quest, and about how valued you would be as a horse. And it serves me best to leave you as smart as I can, so you will be better than any born horse."
Nodding slightly, Mike hesitated a moment, and then leaned over to let Aaron use the brush on his head. The stableboy was still a bit taken aback at having a volunteer, but obliged by slowly drawing out Mike's face into a full muzzle. Mike felt his face stretching out, working his jaw and tongue, and finding that his voice had already become whinnies. His mind was getting simpler, math and other skills fading away as his brain changed, and instincts getting stronger. He knew he should be scared, but besides the conviction that was an elaborate game he would eventually find an escape from, the prospect of actually being a huge, powerful warhorse was both exciting and attractive. If he never got back to his human life, so what? No more school, no dead-end job, no bills, nor responsibilities other than taking care of his master, who would take care of him.
Aaron stopped with the former centaur's head almost, but not fully transformed, and completed changing the last traces of human chest into a thick, muscular neck. He stepped back and admired the massive animal, then raised the brush and said "OK, then. Just a little bit left, and that will change as I make you younger."
Mike nodded his head and twisted around to look at himself as the stableboy began using a different brush. The change was immediate, as Mike had already been fairly young. His body lost size and muscle rapidly, and he felt a surge of energy even as his strength lessened. In a few minutes, he went from towering over the boy to being eye-to-eye. His silky coat covered a gangly, big-boned body that looked familiar to him. It was strange. He had vague memories of being human and of growing up as a horse, but as Aaron continued to groom him, the equine thoughts got stronger while the human past faded even more.
"Good boy." Aaron stroked the colt's forehead with the brush, watching the animal's eyes. They were still bright when he stopped and stepped back. "Are you still in there, centaur?"
The colt heard the sounds but did not understand them, clicking his ears. He wanted to go out and run in the pasture, feeling full of energy and oddly happy. It was not a normal thought, for emotions were simply there, not something he usually considered. Then the thought seemed to short-circuit and he dropped his head to lip at some straw.
Aaron jumped as the Billy Goat suddenly turned back into Liam, and then they both were startled as Tye reappeared. "Fair is fair. Weird. I thought Mike would get scared towards the end, but he is totally happy. Maybe he is smarted than the others - I don't think they are going to do well on this quest." He sighed, and then shrugged. "Hope he turns out great for you. I doubt you or he will ever see me again!"
And then Tye vanished, leaving the two stableboys with their newest charge.