Extend the Story - Add a Room

As you pass by and ignore the girln, you hear her chant something and you can't help but stop and look yourself over, suddenly feeling far too tall.
Without warning, your rump seems to grow out behind you, almost making you fall over from it's size, and continues growing. Just as you're about to topple over, you grow legs from your rear haunches, holding you up as you lean against the tree. Your rear hooves dig into the grass behind you, and suddenly your shoes burst open as your front feet turn into hooves.
Coarse horse fur follows the hooves, flying shockingly quickly up both of your legs and moving onto your now-several-feet-long-underbelly. Staring, perplexed, at this development, you pause, looking over your body, a seeming centaur, before your torso slowly shortens, and shortens, almost being sucked up by the front "shoulders" of your front legs. Your arms, too, dwindle and disappear, your clothes tearing and falling from your body. Your hair falls from your head and disappears, leaving all but a thin strip down the middle that becomes your mane.
"Now you will learn proper manners with others as my new horse

She mounts you