You watch for the horse for the next half an hour, but no one ever shows up. Eventually, the sounds of breaking branches return, and you see the girl step out of the trees, looking sad and holding a bottle. "No luck. Any from you?" She asks, but you shake your head. She sighs. "Well, thank you for all your help. Here, I bought a drink. Do you want it?" She asks, handing the flourescent green sports drink out to you.
You shrug and take it from her. The girl sits down next to you on the bench and watches as you gulp it down. It tastes odd, almost...minerally. You brush off the strange taste and continue until it's all gone. "Uh...thanks." you say, but the girl says nothing. She just continues staring at you. Oddly unnerved, you take out your phone and start playing a game.
Your flat stomach suddenly pushes out. It is subtle, but your skin is shifting under the cloth. You don't seem to notice. As you put your phone down, your belly is beginning to poke out of the bottom of your shirt. You still haven't noticed the changes but as you drop your phone on the ground and bend down to pick it up you do. Your new, still-expanding belly is getting in the way of leaning forward. You freeze in place, shocked into inaction. The girl innocently grabs the bottle from your hand and places it on the bench beside her.
The contact jars you from your reverie, and you slowly and uncertainly reach towards your belly which is still expanding and pushing your shirt further and further up. You make some kind of detached murmur, and that breaks the dam.
"Wh-what's going on?" You finally say, your hands shaking as they rub the soft fat. The question causes the girl to look up to meet your gaze, and you notice that your head and neck have similarly fattened up as well. Now that you look, you notice that your butt is growing fast as well. Your jeans are strained by the mass. You scramble in your seat to get a good foothold on the path, but your balance is thrown off by the unfamiliar weight. As you flail, the unmistakeable sound of ripping fabric tears through the air. You freeze, but only for a moment. You quickly lose balance and fall back onto the bench. Your jeans have torn across the bottom, revealing similarly strained boxers. Another tear rings through the air and you finally find purchase on the path to stand upright. You pull wildly at your clothes like they are on fire and manage to pull your torn jeans and boxers off of your butt, which bulges out towards the girl's face as it is freed from the too-tight clothing, revealing a rear covered with a layer of dark-brown fur and the beginnings of a tail. You pull your strained shirt off over your head and stand naked in front of the girl.
"What did you do to me?!" you gasp at the girl. You hyperventilate as you run your hands over your misshapen, swollen body. Then... the changes begin to get grotesque.
Your arms, which have so far been spared the changes, are getting thinner straighter. They twist around into an uncomfortable-looking position, and your hands get slower and more sluggish as you rub your belly. As your manual dexterity plunges, you finally pull them from your swollen body and look closely. Your fingers change shape, balling into fists and turning hard. You let out a shriek of terror and shake your hands as if something is stuck to them. The blur of movement is too indistinct to make out, and when you hold them still to look at them again, a thick hard material had replaced your entire hand.
"What-" you begin again. "-What is this?!" You turned to the girl, who has been watching silently the whole time. The girl opens her mouth to say something, but you cut her off with more hysterics.
"What is happening to me!" you shriek, your voice suddenly dropping a few octaves. "What's wrong with my haa--!" your panicked voice cracks into an undignified neigh.
"This isn'th whah..." you begin, but you struggle to spit out the words. Your legs are following a different suit of your arms. As the bottom part thins, your thighs add mass, growing to stallion-like proportions along with your torso and hindquarters. Your feet ball up and pop into thick hooves as your spine cracks and you shoot up three feet in height. All along your body, your skin is being replaced with dark brown horse-hide until every inch, even your face and neck, is swamped with it. Your neck bulges in size and pushes your head out grotesquely.
Your misshapen legs strain to hold your massive weight. Suddenly, they snap into different orientation, and you fall to the path in a magnificent tumble, landing on all fours. You whinny in pain.
"Ah cah--" you try, but your mouth juts forward, cutting you off. You try again to speak, but your elongated, unfamiliar mouth prohibits it. All you manage is incoherent vocalizations interspersed with equine neighs.
Your face deforms, nose and lips growing huge, black, and rubbery. You are no longer recognizeable. Your jaw extends out of shape, and your ears grown long and pointed, moving to the top sides of your head. There is pricking down your long neck and top of your head as a mane grows in. You try to stand up, but your new skeleton forces yo to remain on all fours. You try to cry out, but your new voice only produces neighs and whinnys. As your snout pushes out even further and your eyes are forced further apart to the sides of your massive head, the last of your human attributes vanish. A long dark tail flomps out over your large rear and a white diamond pattern appears on the hair on your forehead. The transformation skids to a halt, leaving you a stallion, standing shocked on the path.
The girl smiles.