At first, the tack seemed like a bad idea as it felt uncomfortable and constricting to your movements. But as you walked on, the feeling subsided and you shook the feeling off, unaware that it was due to your body changing to match the equipment, not the other way around.
Stepping became easier as your feet began to stretch out, becoming thinner but sturdier to the bone. Your toes began to fuse together with the nail, becoming darker and hardening into round ended keratin. This combined with your extending feet causes the toes to rip out of your shoes. The rip is audible enough to hear, but you don't connect the sound with the relief felt by your new exposed hoof feet.
Your rear was next as your pants balloon out and strain at the seams, barely holding together. Another constrained feeling grows, this time, it was just above the seat of your pants. It doesn't hurt as much as it looked like, but it was definitely something you wanted free as you leaned against a tree for support.
As you try to figure out what's making this terrible sense of claustrophobia in your butt, you look at your hands as your fingers begin to shift as well; your fingernails begin to thicken into blunt keratin, like your hoof feet, and your fingers begin to merge together. Only this time, you now have three thick keratin tipped fingers and thumb, all movable and more powerful. Oddly enough, even though you witnessed you hand change before your very eyes, it doesn't process through your mind that anything was out of place.
Pressure continues to build in the seat of your pants and the bump clearly grows bigger as changes now focus on your head. Your ears begin to flick at the sounds of nature, crawling up the sides of your head until they rest near the top, pointed and curved in an equine shape. You gag some as your bit briefly feels out of place, only for it to rest snugly a few moments later as your face pushed out into a half-muzzle. Your nostrils widen and huff loudly, letting off some steam due to the frustration caused by the horse bit.
Your eyes stay relatively in place, though you're now able to look 90 degrees to the sides, mimicking a horse's sight as if they were at the sides of your head. Your hair also begins to bunch together, now resembling more of a mohawk that continued down your back, growing underneath your shirt into a luscious black mane. Your whole body begins to tingle as short, smooth brown fur begins pushing out of your skin. Your face feels particularly tingly as a stripe of white fur appeared on your forehead, just like how the girl described.
Your body finishes its overall changes and, as if on cue, pressure suddenly spikes in the seat of your pants, causing a large hole to open, exposing your bubble brown furred butt. But on the plus side, the constraining pressure is finally gone, as your newly grown horse tail swishes happily in the air behind you in all its jet black beauty.
With the feeling gone, you stand back up, wondering what to do next as if the situation beforehand never happened, remaining completely oblivious to your changes. Strangely enough, a familiar voice whispers in your mind a name, "Moonbeam". You wonder why you remember that name, though you do recognize the voice as if you heard it recently...