Mika started grooming the centaur's coat using the special brush. Like the salve, he could feel a slight tingling in the handle as he used it, but nothing happened. he was apparently immune to the magic. Or so he thought until he used his fingers to pull out some loose black hair from the bristles and felt a strange pulling. The fingers he'd used felt pleasantly warm for a few seconds, but also thickened and darkened. Mika's eyes widened and he was careful not to touch the bristles after that.
His misgivings about using the brush on the centaur faded as he worked across the back and rump. Although healthy to start with, the centaur's coat grew out more, silky and shiny. Any remaining blemishes or flaws in the centaur's hide vanished, and occasional white hairs became black. Although he was already an impressive size and seemingly in peak condition, the centaur got slightly taller and the muscles in his body and legs got noticeably larger. The grooming was obviously pleasant, as the centaur relaxed even more and closed his eyes, smiling as Mika worked. That was probably a good thing, for the joint between human torso and horse's body began to change as the Shire body improved. It was so slight a change that Mika almost did not catch it. He was working on the centaur's massive forelegs when he realized that the sleek animal hide that had been just belly had spread higher, now more than halfway up. Mika stared a moment, then looked back at the equine portion that shimmered in the low light like the finest silk. The centaur would not mind a little more horsehide when it looked this good.
Moving under the centaur's equine chest, he continued perfecting the coat, and then paid attention to the already impressive genitals. The centaur grunted and shifted as the bristles tickled the heavy sheath, but remained smiling and drowsy. Still partially erect from cleaning, the massive shaft lengthened and thickened a little more, and the testicles swelled out noticeably to stretch he black sack taught and smooth. Cloudy liquid began to dribble from the thick cap, and Mika rubbed the shaft with his free hand. The centaur grunted again and said "Keep that up and we may have to get married." He sounded amused, and as Mika continued masturbating, said "Gods, boy! I am definitely going to marry you!"
Encouraged, Mike used one hand to slide the foreskin back and forth while the other kept the bristles in contact. The centaur quickly became fully erect, arching under his belly, and rocked slightly on his legs. More fluid dribbled out in a steady stream, getting thicker. The rocking increased as the centaur felt orgasm build, and Mika used his arms and hands to hold the massive penis against his chest, the massive organ sliding against skin made slick with semen. Mika could not see any further changes, but kept the bristles pressed against the shaft as he was lifted up slightly with each thrust. The centaur was grunting like a stallion now, shuddering and thrusting hard enough to make Mika's feet slide over the straw.
Facing back and embracing the massive cock, Mika saw the centaur's testicles start to jerk, and released his hold just as the centaur exploded with a hot, thick jet of semen that splattered Mika's face and chest as he stumbled back out and fell on the floor between the centaur's forelegs. A second jet covered the rest of him with the yellowish equine spunk, and the third and final ejaculation caught him square in the face as he gasped in a breath. Mika crawled back, coughing and swallowing globs of semen, then stood shakily with the sticky fluid dripping from his matted hair, arms, legs, and everything else. He wiped his eyes clear as he said "I never knew anything coul..." then he stopped and stared as black nostrils snuffled curiously at him, and thick equine lips opened to pull and lick at his face. There was no trace of human left in the centaur - the sexual activity had turned him totally into a magnificent Shire stallion. The animal lipped and rubbed his muzzle against Mika with what seemed to be affection, but the huge brown eyes showed no intelligence or thought beyond that of a normal animal.