Matt groaned. His whole body felt flush, and he suddenly had a very pronounced erection straining against his cum-soaked pants. The rooming was spinning, and he felt very lightheaded. A sharp tugging sensation around the base of his cock made him moan. It was like a hot pleasurable fire was warming his flesh.
He watched, amazed, as his pants and underwear slow grew outwards; fighting to contain his thickening penis. The warmth continued to massage him, and he couldn't help the blush that covered his face. His cock slowly grew longer, thicker, and with more male musk than ever before. The human tip pressed up against his bellybutton, but, it didn't stop there. More and more of his length continued to rise and he saw that the human skin color was tainted with dark black spots.
"I... can't... fight... it... anymore!" He reached down and gripped his throbbing meat with both hands, just barely covering the entire width of the mutating cock. He furiously started jacking off, rubbing the dick with expert precision. The mutations continued unhindered.
The skin slowly turned pink, and that dark spots continued to spread over the length of his penis. The tip flared outwards, and was covered with new flesh; it was especially sensitive and he neighed every time one of his fingers brushed against it. The thickness was rapidly expanding, and soon not even two hands were enough to contain it. A medial ring started to form in the middle of his newly formed horse cock.
His horse cock grew, and grew, and grew. Until his was able to lick the head of his formerly human penis.
His ball were not neglected either, they swelled with equine seed and were covered in a dark furry coat. At the base of the equine mating stick, skin started to creep upwards encasing the bare flesh and creating a lovely horse sheath for the new horse's meat.
Then...