You watch with relief as the rubber around your cock and balls gradually lowers in size, tightening down onto your body underneath. The balls deflate quite a bit faster, quickly pressing around your balls and leaving them in a rubbery sack that tightly contours around your own. The rubber around the member takes a little longer... but as it reaches down to the size and shape of your own cock, you feel a tube inside of the nozzle slide itself into the tip of your penis, moving deeper and deeper as the rubbery cock shrinks smaller and smaller. The sensation is so foreign and unpleasant that you reflexively paw at the latex member with one of your hooves... and manage to pop open the nozzle itself.
Just as the hissing of air escaping's finally finished, you start hearing another hissing start. You can feel it this time, the air rushing into your dick, flowing right down to your testes, slowly filling all of it out. You fumble at the nozzle, but your hooves aren't nearly dextrous enough to flip it shut with you in such a panic and the air just keeps filling in. You can only watch as your dick presses back out into that mushroom shape, your balls once again growing large enough to press against your thighs. Worse, you can see their tight leathery black flesh through that gash you'd cut.
Your package reaches the size it had been before, but still shows no sign of stopping. Even at two feet it keeps on growing, that gash ripping larger and larger, showing more and more of your growing genitalia. When your member's reached out to two and a half feet, the gash has already started traveling up its length. Then, finally, as your balls look even larger than those of the animal they belong on and your cock's pulled out to a full three feet, the red rubber covering it snaps back to unveil the fully horse-like member in all its glory, its broad tip instantly splurting a blast of cum that shoots the nozzle clear out of it and leaves a mess on your wall.
Now sitting on the floor of your kitchen, you can't think of what to do now. Your massive black horse junk is the only part of you exposed, your dick laying across the floor between your legs and your enormous balls sitting underneath it, the rest of you still trapped inside that tight red rubber horse suit and still (at least as far as you know) human.