"Wait a minute, Aaron." The boy wizard put a hand on the young man's arm to keep him from continuing the grooming. "This isn't fair to our four-legged friend here. He obviously doesn't know what is going on, and if we don't make sure he understands the truth, we are just as bad as the evildoers you want to fight."
"But he agreed to be my warhorse!" Aaron looked worried, and also sounded reluctant to stop. But he did drop the brush and take a step back. Aaron watched he pair curiously and said "So what is it I don't understand? You are turning me into a Shire warhorse, right? And I will be a real animal until he is done with me, and then I can chose what I want to be after that."
"Well..." The boy wizard flushed slightly. "That is technically true, but the real fact of the matter is you will be a warhorse in all ways long before you can make any choices. And a warhorse doesn't comprehend choices. You won't be able to chose being a centaur, or human, or anything else, because you won't have the intelligence or ability to think beyond food, sex, sleep, and battle. Just like any other trained warhorse."
Aaron frowned. "So I'll be a warhorse forever? No chance of changing back?"
"About that 'forever' - you are a mature warhorse now, which makes you the equivalent of about 30 years old as a human." The boy wizard sighs. "If you survive battles and adventures with Aaron, you'll be ready to retire in another 4 or 5 years, when you are 13 or so. That gives you maybe another five years before you die of old age."
This made the former human teenager blink. "I'll die of old age in ten years? I'd be 28 as a human, just getting into my prime! What happens when I die? Will I go back to my old life like this was a game? Or will I end up in the animal afterlife, if there is one?"
The wizard shrugged. "No idea."
Aaron scowled. "But you promised! And anyway, it's too late to stop now. You can't use your arms very well any more, and the changes we've already made can't be undone. You don't want to be a crippled centaur, do you? Let me make you into a perfect Shire stallion!"