The man injects you with a clear liquid.
"I'm doin' you a favor tough guy. I was amused by your boldness and at least you had the balls to not cry about it, so, you get to be a Centaur Stallion. Trust me, you'll be the herd-master after the next injection. The one I just gave you will make you a Clydesdale Centaur."
The man picks up the second syringe and says,
"This one is for the largest possible male body size and hyper masculine features. You'll have to fight your way up to be herd-master, but with the features you'll be getting, you'll probably do that easily. You'll have the huge balls to match your attitude." The man chuckles, "I don't think there will be a Centaur, male or female, who will be able to reject your sexual advances. Enjoy your new life!"
The man rubs his crotch, you see quite a bulge in his pants. "Yeah, I get off watching guys change into beasts, I'm goin' to the monitor room to watch you on the TV as you change. I figure I'll get a couple, maybe three good jerks while you change. We got camera coverage from every angle."
He rubs his package again and you notice the growing wet spot between his legs. "You make some good porn for me, now, OK?" He laughs lustfully and walks out of the room.
You feel strange, hot, achy. You feel your spine lengthening and between your pelvis and lower back you feel new limbs forming inside of your gut. Your feet are becoming hooves and up your legs black horsehair is beginning to cover your changing legs and thighs. Your cock is growing into an equine shape and becoming much thicker than any horse cock you've ever seen and you feel your balls swelling. These must be the hyper masculine features you guess. You know you're being watched, but you can't help yourself, you reach down and begin jacking yourself. It feels so good and you're so large now. You rub your cock with two hands and begin to moan. The sexual activity is brought on by the transformation and the transformation is increasing speed with your sexual activity. As you shoot huge gobs out you realize your balls probably shot their last human load, quickly followed by volleys of Centaur sperm. As your orgasm subsides, you spasm in pain as freakish muscle is builds over your torso. These new muscles are quickly covered by human chest hair which becomes a thick carpet that grows down to your belly and up over your shoulders. Your arms become covered in curly human hair and you feel that same hair in growing down your back. Your beard thickens and grows out curly and black to match the hair on your torso and forming lower body.
As two hooves begin to emerge from the growing space between your pelvis and lower back, you pass out from the intense pain. When you awake...