...a small black cat. You're a bit surprised the girl isn't here, but follow the cat inside and shut the door behind you. Once it's closed, the cat stares up at you and to your amazement, it slowly transforms.
It picks itself up on its hind paws like a dog, and its stance seems to alter to be bipedal. Its back legs are growing larger and stronger, and its front paws seem to develop and turn more dexterous. Its fur is receding all over, and the skin at its roots looks oddly similar to a human. In fact, as it starts to completely vanish in places, you realize it IS a human's skin. Soon it seems to adopt feminine traits, as its hips widen and its chest expands to form large soft breasts, and its head reshapes and its fur vanishes- at which point you recognize the goth girl, whose leftover fur around the top of her head flows downwards and turns from black to brown. The last few anthropomorphic traits of her body, her feline ears and tail, now fade away, and she stands before you naked, amused by your astonished reaction.
"Liked what you saw?" she giggles. You nod, still dumbfounded. "I'm a shapeshifter," she explains. "I don't know how, but ever since I was a little girl, I've had this power to think of turning into something and my body just... doing what I say."
"That's so cool," you reply at last.
"Thank you," she replies. "Not only that, whenever I want someone else to be able to change with me, their bodies can change too. Something about you made me feel like you might like to try..."
"I'd love to," you agree.
"Then what should we turn you into first?" she purrs.
(A/N: obviously feel free to add more animals or creatures!)