The boy gaped at his huge, black shaft and reached down to feel it, then rub it as he pounded erect. He was so busy masturbating the thickening penci that he didn't notice other changes that were sweeping over his body. The dark hair got dense and shaggy as it spread, turning pink skin into animal hide. He was getting taller, more muscular with every heartbeat, now well over six feet tall and shooting up visibly.
By now, the boy had a massive sheath around the base of his monster cock, and testicles the size of tangerines hanging between deep, furry thighs. The shape of his penis was no longer human - the tip had flattened and flared out, and those in the audience who knew horses recognized the equipment of a stallion.
In another minute, even the most ignorant person in the audience knew what the boy was becoming - his face had pushed out into a recognizable muzzle, ears becoming pointed and shifting up besides enlarged eyes. His chest was barreled out and he was having trouble bending his arms to the task at hand. It wasn't until his fingers started merginging into dark lumps, however, that the boy finally shook off the intense sexual daze and realized just what was happening to him.
A look of shock flickered over his face, and then human emotions became unreadable as his head finished becoming that of a mature Shire stallion. He fell forward heavily, making the ground shake and the stage crack as his massive forehooves took his weight for the first time, and he gave a loud whinny as the final ripple of transformation completed.
The audience stared in a mixture of horror and awe - the boy appeared to be completely changed into a massive black horse. Madame Illusia walked around him, checking him over like a farmer exmaining a potential plow horse. "Excellent! You are now bigger and stronger than any man. And most horses. Does this meet your desires and expectations?"
The horse snorted, and twisted its head around to look back at itself. A few shakes of its hindquarters brought the black firehose back to full erection, and the animal whinnied again as a spray of horse cum erupted. After it had pumped a last spray of thick white liquid onto the stage, the former boy shook his head, worked his jaw a moment, and then slowly and deliberatly nodded.
"I am glad you are happy. Since this is your new form for the rest of your life." Illusia smiled and motioned for a stage hand to lead the animal off. "He is a young man no longer - but an intelligent work animal who will serve me well as both draft horse and stud. His life may be somewhat shorter now, but he will find contentment that he would never had enjoyed as a human. Does anyone here want to join him?"