Mike was already swinging hard, and seeing the teen teasing Nightmare's cunt with his fingers had the centaur almost too aroused to control himself. Both the younger boys moved back, giggling. Mike heard one of them ask if the magic was still active, and the other responded "We'll find out."
Active magic? Mike frowned. The only magic he'd experienced had been what Liam had done to him in the stable. Making him a huge, beautiful black centaur. Then he remembered the so-recent mounting of the Shire mare in town and shivered in anticipation, his suspicions forgotten as lust made his blood boil and his balls churn.
Caleb chuckled and held his slick fingers up to Mike's nostrils. The scent was too much, and the centaur was overcome by stallion's needs and instincts. Only dully aware of the shouts of encouragement from the boys around him, Mike found himself mounting a Shire mare for the second time in as many hours. The first thrust missed, but he found her opening on the next thrust. As he slid forward over her rump and drove in deep, a ripple of heat and pressure passed through his body from rump to the top of his head. The sensation was repeated with each thrust, merging with the intense animal needs and pleasure.
Caleb and the boys watched as the huge black centaur got even larger, at least in his equine portions. Already a mature young stallion, the centaur's equine parts got thicker and stronger as he aged to match the mare. At the same time, his face pushed out and his upper body shrank. Mating horses would normally be done in a minute, but Caleb stepped up behind the rutting animals and massaged Mike's testicles to keep him going.
Mike shuddered in release, but instead of breaking off, felt the sudden recharge of his seed and continued on. The mare whinnied in pleasure, and he grunted and whinnied himself. His arms flapped uselessly now, boneless bags of muscle and flesh that were drawing into what was rapidly assuming the form of a horse's neck. His nostrils flared, now fully equine, and his eyes showed whites all around brown liquid horse's eyes. He knew he was becoming a real horse now, could feel his perspectives change, his head grow heavy, his very soul simplifying. But he could not stop himself. It wasn't until every visible trace of centaur was gone and the animal mounting Nightmare was a ten year-old Shire stallion that Caleb stepped back and let the animals complete their mating.
Mike shuddered in final release, and slid off the mare's back to recover, his hide lathered and breathing hard. The mare snuffled and nuzzled him, and he and she stood together for a while. He gradually became aware of the boys, still watching him with grins on their faces. His mind felt oddly thick and a little confused, but mostly content. The older of the three came over and gave him a nice chin rub. "Good old boy. We'll get a good eight to ten years out of you."
Eight to ten years? Mike struggled to comprehend what the boy was saying, but he wasn't sure what a year was anymore.
Caleb grinned as he and the boys left the former centaur to rest. They could put him to work in the fields tomorrow. He'd be a natural plow horse. Too bad Liam couldn't have finished him up with the young mare in town - they'd only get half the life span out of him. But then there were always centaurs to change, so it didn't matter that the formerly young centaur was now a middle-aged horse.