Mika handed the brush to the centaur. "I'd like to be your horse. A drafter, please." The centaur smiled and nodded, and ran the bristles over Mike's forehead. Nothing happened. A few more tries sowed that the brush would not work on him. The centaur shook his head. "I guess that option is out."
"Maybe not." Mika frowned, trying to figure out why the transformation magic wasn't working. "You don't really want to make me a horse, do you?"
The centaur frowned. "Well, not really. You're a nice kid. And you'll probably not like me when you are a stallion. We'll be too much like rivals."
Mika thought about that a moment, then grinned. "I think you just need a little bit of encouragement. And maybe some change to work with." He ran the brush over his belly and crotch a few times, concentrating hard. The centaur watched curiously, then peered closer as a dark area began to appear on Mika's belly, and his male parts started to shrink. Mika felt his insides pulling and shifting as he worked, and felt dampness and warmth under his anus. When he handed the brush back to the centaur, he had the udder of a mare low on his belly, and a black-lipped mare's cunt that clenched and released every time he looked at the powerful male centaur.
The centaur's nostrils flared out and his eyes widened. "You re in heat!"
Mika nodded. "If I have done this right, I will be receptive to mating all the time. How is that for incentive?"
"But.." The centaur flushed, but grinned. He reached forward with the brush and ran it over Mika's forehead again. This time the result was immediate. Mika's forehead thickened and then sank in, and his brows shifted. Two more strokes and the former boy was no longer recognizable. Black hair was starting to spread over his body, and the centaur shifted and leaned over to run the magic bristled over Mika's chest and arms. Mike felt his whole body enlarging, bones shifting. Would he remember being a wizard? It depended on what the centaur wanted. From the intensity of the changes, he could tell the centaur was focused and his desire was clear. The transformation was moving rapidly, and Mika was forced to lean forward as his weight shifted. As HER weight shifted. She realized that she had not been male since she created the slick equine cunt. The centaur was the male here, his shaft already pounding out.
So heavy, so big. Mika shook herself as her body formed, arms becoming forelegs, a massive, barreled chest, powerful muscles. The centaur finished up the mare's body and neck before moving back to face Mika. The former wizard's head was halfway between human and horse, enlarged to stay proportional. The centaur was fully erect now, eyes wide and face flushed with desire. He ran the bristles over Mika's face, drawing it further out into a muzzle. As he did, Mika felt a curious pressure, a tingling in her head. It was her magic, the ability to cast spells, the source of her power. The brush was starting to pull it out of her head, absorbing the mystical energy with each stroke. Mika would no longer be a wizard, have no magic at all. She would be Clydesdale mare, property of the centaur.
Mika felt fear stirring as her muzzle formed, the last traces of human form being lost. The physical transformation completed, Mika now indistinguishable from a born horse. There was still some magic in her head, but more than half was gone and it was drawn out faster now that the brush only had to adjust her mind. The centaur was panting with desire now, Mika's scent screaming to what was now a young stallion's need. And the centaur's musk had Mika excited and feeling a similar need. Fear grew as Mika realized the combined desire was influencing the change - they both wanted to mate, and the act of mating was based on animal instincts. Using the brush on the centaur had made him horsey looking, and the magic must have worked faster on his mind. He still had his memories and intelligence, but Mika realized the centaur now had the core behavior, the soul of a horse. This was what Robin had wanted her to do to others. Now she would become an animal, a beast forever.
She felt a momentary surge of panic as the last of her magic was absorbed by the brush, but then her thoughts were overwhelmed by the stallion's scent and her own heat. Mika shuddered and urinated in front of the male, dropping her head in submission. Instincts told her what to do, and the powerful male had the instincts of a young stallion in command. The brush fell from the centaur's hand and he moved to cover her. She felt his first clumsy attempt, his weight pressing over her rump and back, two prods, and then the intense pleasure as he drove in.