Mike frowned, so worried that he was starting to panic. "I don't want to just be a dumb animal, and I certainly don't want to die in ten years! Is there anything else we can do, what other options are there?"
"It would not be easy," said the wizard, meeting Mike's alarmed frown with a more contemplative one of his own. "Remember, your reality has been changed. As far as the universe is concerned, you really are a horse, and always have been. Left unattended, your mind and soul will gradually become those of a horse, though probably more slowly than they would if you had the horse's form too. Because of the way your timestreams have been crossed, I cannot return you to being a centaur, or a human, and beyond that... well, it is one thing to transform a sentient being into an animal, and another thing entirely to go the other way around. As I have said, the changes we have made cannot be unmade, in a sense. You must remain partially an animal. But you could become another sort of hybrid, like a minotaur or satyr. Getting rid of all the animal and transforming you all the way into, say, an elf or a dwarf would require much more magical power than I currently have at my disposal. I would need proper reagents, for one thing."
Aaron seemed to perk up at the mention of reagents, and suggested, with an uncomfortably crafty note in his voice, "If you turned him the full way into a horse I could take him off on a quest to get whatever you need for your spells and then bring them back here, so you can turn him into whatever he wants once I'm done with him. Everybody wins, right?"
The wizard crossed his arms and looked suspiciously at Aaron. "Perhaps. But he would have to agree to it. And do not even think of trying anything untoward. If you were to lie to me and fail to bring him back, well, transforming into a horse would seem like the most generous reward in the world compared to what I would do to you."
Based on the alarmed look Aaron got, Mike did not doubt that the wizard would do something horrible if crossed. And then it was Mike's turn to be alarmed as the wizard turned to him, though it was only to ask, "So what will it be, centaur?" Mike had to think for a good long while before answering...