Aaron sighed and looked at the billy goat now and the partially-transformed centaur, then frowned. They were not moving at all. Tye grinned and said "It would be unfair for Mike to see me. So I am giving you a chance to talk him into being a warhorse, or a draft horse. He won't remember the other boy, just the goat. And the goat is a contented animal, no longer your transformed friend. Finish him up against his will and you are the new head stableboy with a goat to take care of. Get his cooperation and you and your buddy are both fully human again. Fail both and the centaur leaves as he came in and there will be a Billy goat and a soon-to-be pregnant nanny goat in here."
"Fair enough." Aaron moved to stand in front of Mike and nodded. Instantly, the mishapen centaur was moving again, shaking his head and trying to work his nearly useless arms.
"Yuuh.. you're turn me inuh orse?" The stableboy almost smiled at the repetition. But he also noted that the centaur was not actually angry - more curious, if showing concern. Nodding, Aaron said "That's what you wanted, right? To be a beautiful, perfect Clydesdale stallion? Even better, a colt just starting his training, a whole new life ahead of you full of mares, contentment, great strength and power?"
"No!" Mike bristled, but then looked uncertain. "I mean, I wanted to experiment. Maybe not all the way to a horse. I guess I knew what you were doing to me, but..." He hesitated, then twisted around to look at himself again. "A Clyde colt? How old?"
"Ideally?" Aaron shrugged. "Maybe 9 months old, intelligent and curious, just starting to grow into a powerful stallion." He could see the centaur nod slightly, obviously liking the idea. "Centaurs can't go into most dungeons because you are too big, and half the time the party is going to use you like a pack animal because you have the body for it. When you are not on an adventure, you'll have to use stables because Inns can't house you. And there's the real biggie - unlike a warhorse, which would be prized and have great value to anyone, a centaur is just a big vulnerable target for the baddies."
"I don't want to forget who I am, or was." Mike had to agree with the stableboy - a centaur was a terrible choice for adventuring. He'd selected the form because he liked being turned into a horse, and the DM usually accommodated him. Except this was a lot different from role-playing a shape change. Or was it? All of this felt real and looked real, but it had to be some sort of mind game Tye was playing. Some hyper-realistic virtual reality? Did it matter? "But that sounds pretty cool."
Aaron nodded and held up the brush in his hand. "I promise you will know exactly who and what you are, and that being a Clydesdale will be the best life you can imagine." That was because Mike would have no concept of being anything other than a horse, and horses lacked the cognitive ability to imagine. However, the centaur did not have to be told that. He approached Mike and reached up carefully, slowly bringing the bristles in contact with the centaur's chest. When Mike did not pull away, he started slow, even strokes that started shrinking the chest and arms even as the golden coat of a Clydesdale spread upward.
Mike was looking more like a misshapen horse now, little evidence that he had ever been a centaur left. If Aaron was to do this properly, he needed to work the mental changes soon. "OK. Lean over as far as you can. I'll give you the full head of a Clyde - even the instincts and memories, awareness. You will find out what it is like to really be a horse." This was total disclosure - would the centaur read between the lines?