Your mouth slams shut just as quickly as it opened as you realize that isn't your voice. It's far too high pitched, and you just said buck! That's a pony swear! Okay, deep breaths. Work out what the hell has happened to you now, freak out later.
You lie there on your side for a while, just staring at that round, grey thing hanging where your hand should be. You have a pretty good idea what it is, but just... How? You flex the 'arm' experimentally, your heart sinking as you feel the lack of fingers. You wave it about in front of your face, noting that it's very much your own limb. You throw your duvet to the floor with a flick of your other... hoof and you crane your freakishly long neck around to see what else has happened overnight.
Lying on the bed in front of you in the place your body should be sits a small, grey cartoon pony. Your jaw hangs open and your pupils shrink rapidly as you stare at it with one eye, the other still fixed on your strange new appendage.
Wait, what?
And now you're back on the edge of freaking out. It's clear what's happened to you, and it seriously worries you, but this? You're not sure if you can take this. With considerable effort you manage to focus on the end your nose, which you suppose is now a muzzle, if you want to use the correct terms. You watch your new pony belly rise and fall as you breathe deeply, and allow yourself a moment to think. So you're Derpy now. That's cool. No! This most certainly is not 'cool'! What will your parents think? What will anyone think? You're a cartoon character from a show for little girls for Celestia's sake!
You've tried to avoid it, but there's no stopping it now. You fold your legs against your body and curl into a tight ball, shaking uncontrollably as the full realization of what's happened to you hits you in full force. This isn't your body. You can't be a pony. It's not possible! Large, soaking wet tears roll down the sides of your muzzle from your huge, misaligned eyes.
You don't know how long you lie there for, but eventually you manage to pull your thoughts together. You may not be human any more, but you're not just going to sit here and wallow in your own self-pity for the rest of your life. It's clear now what the plushie's purpose was, but you're still worried about why this happened to you of all people. You have more important problems than that right now however, and your attention turns to thoughts of getting something to eat. Whatever crazy voodoo magic that plushie had on it, you can worry about it later. A heavy sigh escapes your mouth as your tongue hangs amazingly far out the side of your mouth, fond memories all the delicious foods your new pony stomach won't be able to digest flashing before your eyes. Bacon, burgers, buckets of fried chicken...
Oh, why must you torture yourself like this? It's bad enough not being able to look in specific directions, but over-thinking things like this isn't going to help anything. Okay, next order of business: find something to eat.
You shuffle to the edge of the bed and stare apprehensively at the floor, noting how much larger everything has become. You raise your right foreleg and tentatively lean forward, the tip your hoof making contact with the hard, smooth surface of a floorboard. You gradually shift more and more of your weight to the hoof before reaching down to do the same with your left foreleg, leaving only your hind legs balanced precariously on the edge of your bed as you realize you don't know what to do now.
As you ponder your next move, however, your forelegs slide out from under you, your hind legs dropping painfully onto the floor and your face slamming into the floorboards. Welp, that's one way to do it.