The fur takes on a white shade as it quickly sweeps up your rapidly pumping arm, which begins to swell out. Your hands darken and turn kind of thick, a cross between hooves and hands. This doesn't make your masturbating any harder, however, as your dick swells to stallion-like proportions, a foot and a half long and swamped with white fur.
As the white fur make it's way up your neck and down your stomach, it leaves bulging muscle in it's wake. Your pecs and abs stand out and your arms are at least a foot around. Your ass swells to fit the confines of the diaper, and the pressure on your tailbone gives way to a huge, long, flopping golden tail that hangs out of a hole in your diaper. Without thinking you lift your tail and blow another load of scat, some of it catching in your tail as it hits the ground with a plop.
Your legs crack and become thick, muscular, and horse-like, while your feet ball up and harden to become hooves that clop on the floor while you continue to jerk off. Your spine stretches until you're eight feet tall and your muscles become larger again. You look rather odd now, with a small human head on a massive anthro stallion's body, but that quickly changes as the fur bursts your neck with muscle and shoves it out. Your hair falls out as a thick golden mane grows down your neck and your ears stretch and move to the top sides of your head. Your face bulges out with so much force you phisically feel your dumbing eyes being moved to the sides of your massive head. Your lips and nose grow huge and rubbery, moving to the front of your face as your tongue thickens and teeth become huge and square.
You throw your head back and neigh loudly, your body giving everything a final bulge outward as your dick thickens and stands straight out of your brief, spraying a huge load of cum onto the wall next to the box.
Whinnying softly, you paw at the ground with your hooves as you look over yourself, pleased with the results. You clop your way over to the door and open it with some difficultly. You leave, but not before grabbing a few of the briefs for your friends.
You try and look around for the beats with the briefs you saw earlier, but see none. You neigh and shake your head, flopping your mane in frustration. Finally, you decide to just leave. You turn to make your way downstairs when you feel a huge, beefy hand clamp down on your shoulder. With a whinny of fright, you are turned around to be greeted face-to-face with an anthro German Shepherd, his huge muscular form just about a foot shorter than yours. Looking down, you see that he is wearing a beast brief as well, his tail hanging limply out the back.
You gulp as the dog looks up at you, clears his throat and says, "Hey...