"The next time you cum, the seed is coming from the balls of a Clydesdale!"
You stare at the guy. A Clydesdale? One of those big horses that pull beer wagons, right? You'd shout 'bullshit,' but your face is already changed from toothpaste. Instead, you ask "Why? What did I ever do to you, man?"
The nurse laughs. "You came in here. That makes you fair game!"
"But my DAD brought me in here! I didn't want to come. I didn't know it was anything but a routine dental checkup!"
The doctor grins. "Oh, well, that was the rpice for your dad to get what he wanted. He traded you for the chance to be lion."
"WHAT!?" You don't want to beleive it, but your dad has always had a thing for lions. "He wouldn't do that!"
The doctor chuckles and picks up the big syringe. "Well, he's well on his way to becoming a total male lion, and you are about to start your journey to equine life."
"No!" You stare at the needle. "The nurse said he wouldn't use that if I let him brush my teeth!"
"Oh, HE won't use it on you." The doctor presses the tip of the needle against the tip of your erect cock. "I suggest you hold still, or this is going to hurt really really bad."
You go rigid, staring in horror as he slides the needle into your piss slit and down your penis. You can feel the cold steel inside, pushing down. He gets it almost all the way in before he gets resistance, and then he shoves it hard!
You SCREAM as he injects the contents of the hypo directly into your balls. It's like liquid fire, and he jerks the needle out as you start to thrash and jerk against the straps. The fire spread up your cock, and into your groin. You are crying, screaming, cursing. Pressure builds inside your cock, and you look down to see it already doubled in size and nearly black. As you watch, the outer skin separates and slides back from the shaft as a foreskin, which then thickens into a sheath attached to your belly.
Your balls are now like black tangerines, and as the pain and burning fades, your cock flattens out into a true horse's penis. The pressure suddenly explodes from your cock as a good pint of warm, thick horse cum that splatters your face and chest.
The nurse massages the still-erect horse cock. "Magnificent! You are gonna make one amazing stallion!"
The masturbation feels incredible - you are terrified, but the pleasure from your huge new organ is distracting. "Am I...." You gasp and shoot another load. "Am I gonna end up as a real horse?"