The clerk grins and says "That looks like a perfect fit! You are going to be such a stud!"
Shaking your sticky, sodden shirt, you nod. "Yeah, I already proved that." Then you frown as your pants and shoes get tighter. "I feel really weird." The tightness of your pants is starting to get painful, and you can feel pressure around your feet building.
"Yeah, well what did you expect? What kind of horse wears pants and shoes?" The clerk shakes his head. "Not that you'll have them on much longer." He looks smaller. "Besides, why would you cover up that beautiful black cock?"
You start to protest, but then grunt in pain as you swell even larger, and your pants dig hard and deep before mercifully tearing apart. The relief is so great you whinny and shake yourself - huh? You now tower over the clerk, at least 7 feet tall and still growing, and your horse-man look is more horse than man. Massive black hooves have exploded out of your shoes, and as the last scraps of clothing fall off, you find that you look more like a powerful stallion raised up on its hind hooves. Your fingers have turned hard and dark, merging into three digits on each huge hand, and the head on your shoulders is totally animal. Same for the huge stallion's cock and swollen testicles, which are pumping up for another firehose blast of semen.