As you enter the shop, you see a horse bridle laying on display. You decide to put it around your face, just for fun. The bridle obviously doesn't fit at all. You chuckle, and then try to bite the metal bit, just to see. As your teeth touch the metal bit, they stick in place, and instantly you feel your face pushing out. As it lengthens considerably, it becomes white and very shiny, and you feel your mouth starting to melt. As your face turns into a long, smooth equine muzzle, your mouth seems to resolidify around the metal bit, and the straps get very tight as your muzzle fills the bridle. You try touch what was you mouth, but you realize it is not completely gone: the metal bit goes right through your equine muzzle, nothing left of the mouth, so it is now totally impossible to take it out. The material feels very smooth and soft - in fact, it feels like your muzzle has turned into rubber. You try pull on your black leather straps, to no avail: they seem to be merged into you now. Then, you see the blinders going down, and curiously you seem to be able to magically see through them, although they still hide your eyes to other people, making you look anonymous and emotionless. As your neck turns into smooth, white rubber, you feel a "choker" strap tighten around it a bit.
You take off your shirt to see the transformation go down your torso, making it look muscular but very smooth and shiny. The changes start going down your arms and your belly, and you take off your pants and underwear to see the changes. Soon, it reaches your hands, and your fingers start to melt together and darken, smoothing out into black, shiny hooves. Horseshoes magically form themselves over your hooves. Meanwhile, the change goes down to your cock and balls, both of which grow a lot, turning into jet black rubber, smoothing out into almost a toy. The change continues downwards, making your legs much more muscular, and then reaches your feet, which morph into jet black rubber hooves just like the hands, with horseshoes forming under them, leaving you standing in a strange but stable unguligrade way. Finally, your rump transforms, your ass becoming quite deformed as a tail forms over it.
All in all, you seem to have gained a foot in height. You are now a smooth white rubber anthro horse, although your black bits seem to stand out a lot.