Mike grinned, desire stronger than his vague concerns. "Sure! If she's willing, I sure am!"
The boy chuckled. "This time will be perfect, I promise. When you are done, you won't be able to imagine ever having better sex."
Already getting rigid, Mike shifted beside the mare. She rubbed against him, still slick and hot, and her scents no longer needed enhancement. He drank in her odor, nostrils flaring, and slid over her back again. This time it was easier to find the mark, and he whinnied in pleasure as he plunged in.
Whinnied? A stab of confusion turned to fear as Mike suddenly realized his face was pushing out into a muzzle. He thrust deep, then pulled back to thrust again, unable to stop. His arms, already shrunk to those fo a child, dwindled away into what was becoming a horse's neck. He thrust again, the last traces of centaur fading from his body, even that altering to match the filly he mounted. Again. The pressure built.
The stableboy grinning at him as he pulled back for a final thrust before release. "Horses don't have imaginations."
Mike knew he had to stop, to hold back this last thrust. But of course, he was a stallion and that was what a stallion did.
The boy watched the horses complete the mating, and grinned as the new stallion lipped at his filly. The matched pair would bring a great price.