The wizard smiled, spreading his arms and asking again. "So, do you want to become a stallion?"
Mike bit his lower lip then shook his head sadly. "Sorry, but no, I can't."
The wizard shrugged, then pointed his staff at Mike, who was suddenly engulfed in cold, tingling fog. He gasped and struggled, but his limbs seemed frozen and he could do nothing more than twitch and shake a little. The wizard hummed as he focused the energies on Mike's body. "Very well, no stallion then. A mare it is."
Brown fur tingled and itched Mike as it started to spread up his body, pushing out from his skin and rapidly covering his bare chest and head in thick, tough horse hide. He whinned loudly, gritting his teeth as they grew large and flat, his nose and mouth stretching and cracking forward. His nostrils flared wide as he grew a long equine muzzle, the fur on either side of his growing face striped with white. His black hair grew in thickly, falling down in long locks down his shoulders and back as his neck bulged with muscle, stretching and growing longer.
Mike felt his ears twitch and push up, growing fur as his eyes watered painfully, moving apart as they shifted to the sides of his head. His chest began to feel strange, and his arms would not move at all. They were beginning to shrink and thin, pulling up into his torso slowly. As they shrank, they were forced against his sides, before the flesh there fused into one, his arms disappearing completely into his furry torso. His ribcage crackled and softened, his chest sinking down into the strong base of his horse body, becoming a strong equine neck.
He could feel his thick, powerful equine cock flop down over his heavy black balls, but it was beginning to change. It hardened and throbbed, but even as it grew erect, it shrank, pulling down slowly into his sheath. The sweaty dangling horse balls between his legs felt tight and constricted as they pulled up into his crotch, shrinking faster. He whinned again as his balls inverted, sucked up into his body, where they moved deep into him and began to transform into a fertile equine womb. His dick pulled down into the hole left by his balls, which grew warm and wet, changing into a warm, sensitive mare's genitals.
Finally, his terror and anger began to fade, strange new instincts rising in him. He could still remember being human, and being a centaur, but the memories did not seem important or particularly pressing. He flicked one ear and shook his tail, scaring off a fly as his thoughts slowed into a thick, sluggish soup of bestial instincts and calm horse-thoughts. His eyes, once sparkling with human intelligence, turned a dark brown, becoming the soft orbs of a natural horse.
The wizard pulled his staff back, freeing Mike, and Aaron leaned down to pat the new mare's shining, strong neck. Mike felt perfectly content in his new body, eager and willing to obey her knight and master for her entire life.