Mike looked back, curious. "How are you going to change me without mare?"
"Oh, I don't need a mare to get a stallion off." Brian grinned. "I got a lot of new skills when I got changed into Dar. I think you'll really enjoy this." He moved around to Mike's side. letting his hand slide around the animal rump and thigh, never losing contact. "Just don't kick me, please." He slid his hand back to the ballsack from the front, and then ran it up the length of Mike's heavy sheath. The thick black penis inside began to push out, still encased in its foreskin.
Mike closed his eyes and smiled. "That feels pretty good. Not like the mares, though."
"Give me a minute here." Brian got up under Mike's belly, facing backwards. He slid his fingers into the sheath and began to lick and lip the flat, flared head of Mike's horsecock, tonguing the piss slit. As he did, he also released some magical energy that stimulated Mike's equine pleasure centers. It was cheating, but Brian was finding out that much of his new life involved trickery and deceit. In this case it didn't really matter, since he had told Mike what he was going to do. And if the centaur was willing to become a beast of burden, Brain was more than happy to perform the transformation.
Mike grunted and then gasped as he was hit by the powerful sensual magic. "Oh, man, what are you doing down there?" In addition to a flood of equine instincts and desires, the centaur felt a ripple of heat flow outward from his cock. The warmth spread down his hind legs, up his lower body and down the forelegs, and then up into his mostly human upper body, arms and head. He felt the prickle of hair as it spread upward from his waist, and could feel his face slowly swelling forward.
Although Mike could not see it, Brian was also changing. His lips had thickened and grown outward to form a fleshy tube that encased the end of Mike's black shaft, and his tongue had swollen and lengthened as well. The boy's mouth met the end of Mike's foreskin and the two tubes of flesh merged. Both Brian and Mike shuddered as the boy became a sensory extension of Mike's sheath, the tube of his mouth secreting a slick fluid that lubricated and stimulated Mike's penis.
Bracing himself against Mike's hind legs, Brian found he could manipulate the fleshy tube almost like an elephant's trunk, using new muscles within the length to squeeze and rub the massive horse cock within it. He was getting intense sexual pleasure from the tube as well, and felt his own human organ come erect.
The changes were slow at first. Although it was hard to concentrate on anything but the sexual stimulation, Mike could see the separation between human skin and horse hide moving higher up, and see the mass between his eyes expand slowly into a muzzle. His arms were thinner and small,, and his whole upper body was feeling strange - not quite numb, but missing many internal sensations. Everything in his upper body was becoming muscle, with the organ functions being taken over completely by the horse guts.
Mike was fully erect now, his cock rigid within Brain's mouth-tube. Brian could feel the pressure building at the base of Mike's shaft, and started a rippling muscle clench around the swollen penis that caused the changing centaur to cry out in a human sound that caught in his throat and turned into something more like an equine whinny as the first release came.
Brian felt the explosion of heavy cum and had a momentary panic that he could choke, but as the gush shot into his mouth, the back of his throat closed off and it was diverted down... his eyes widened as he felt his own testicles swell. Then came another gush, and another, and another. Brian could feel his testicles swell enormously, and still Mike came.
The former centaur shuddered with each ejaculation, being hit with primary instincts and magical enhancement. His body changed rapidly with each jet of thick cum, arms thinning and getting shorter, his chest and shoulders narrowing, his head getting heavier and broader. He could no longer talk, and felt confused by the barrage of intense sensations both from the masturbation and from his own changing body. Seven, eight, nine orgasms. Mike felt solid and heavy, his awareness altering to fit his new shape. There was almost no trace of the centaur he had been now, and after he shuddered for the tenth and eleventh releases, Mike was physically a young Shire stallion in every respect. And still Brian continued. It took longer for the twelfth orgasm, and when it came Mike felt thick-headed and confused. His mind was going! He wanted to stop, wanted to protest, but Brian was already working him up to the thirteenth and final release.
Brian was in a daze as well, feeling intense pleasure from the repeated orgasms as well as a very strange but pleasant heaviness in his groin. He felt the change finish up Mike's body, and savored the rush of intelligence and memories that were part of the next to last thick jet. As he concentrated on this last orgasm, he focused on Mike's soul. This was the tricky part. He wanted Mike to stay Mike, but as a Shire, not as a transformed centaur or human. An animal spirit or essence. He felt the pressure building.
Mike whinnied and stomped his hooves as the last pulse of stallion cum shot from him into the human. They remained connected for a couple of minutes, both sweating form the prolonged sexual experience. Finally, Brian's mouth-tube separated and slid off, allowing Mike's overworked cock to slide back into its sheath. The stallion whuffed and looked back to snuffle at the curious creature that came out from under it.
Brian had to drag his ballsack across the stall, but the skin had thickened to allow that with no real discomfort. That was a good thing, because he now had testicles bigger than large beach balls that were swollen with horse cum. All of the semen he had gotten from Mike was now sloshing inside, and he had to drag what felt like fifty pounds between his legs. More curiously, his mouth-trunk had not changed back yet. He needed to do something with it, but what?