You reach down and set the second largest one out there. (The largest being the elephant) Out of anything, you figure this one will actually fit. When you slip the white ursine muzzle on you, it actually fits but in fact, a little too well. It manages to cover up your mouth, and go down to your chin and bulges out quite a bit.
"Hmmm, maybe it's a little too snug," you say to yourself, voice muffled by the muzzle, "Smells kind of like plas-" All of a sudden, you can't find yourself breathing or speaking anymore. You try desperately to suck in some breath, anything, but you can't.
In your panic, you reach out and pull on the muzzle but it feels like you're pulling on it...like it's part of your own skin. Suddenly, you feel cool air going in through your nose, and you breathe a sigh of relief, even though you can't seem to breathe through your mouth. At least you can breathe.
But wait a sec, touching it again you feel...fur. Yes, fur. You make a noise trying to talk, but all it comes out is like your mouth is gagged. Then your skin starts to itch. Looking down at the muzzle breaking your vision...you see white fur. It's actually growing all over your skin, and rapidly.
You could do nothing as your entire skin is covered in fur which had hardened into hide underneath. When it reaches your hands and feet, you feel your palms and fingers ballooning up when leathery paw pads make themselves known. Out from your shoes and hands you see long black claws come out. Still gagged, you make a very loud noise when your ears suddenly move to the top of your head and your entire body balloons out.
You seem to have nearly tripled in size, and your clothes were no match for how big your body had become. and yet your mouth still feels like it's glued shut, before finally you manage to open your mouth and growl, revealing that your teeth had morphed into vicious carnivorous teeth. You can't even tell that you had been human, just a massive polar bear. How are you going to undo this?!