Of the options, you didn't really care much what you were choosing so you clicked on the first one you could. That just so happened to be be rat.
"Are you sure you want to select this animal?" the screen asked.
You click away, more than happy to just get through the screen. There is a clunk noise and a slot opens up on the machine. Into this slot slides something covered in a tight wrapping. Taking this, you unravel it and find that you're left with a small wheel of cheese. Perhaps Gouda but you're only sure when you bring it to your nose to sniff.
"How nice," you think, taking a casual bite from the wheel. It's smoky and surprisingly good so you take another bite. It takes only one more for the entire cheese to be devoured and you to belch in satisfaction.
Suddenly there is a knocking noise further down in the room you're in. You see a slat open near the floor and something scurries out. It's then that you feel your first shock of pain in your gut.
You moan loudly before belching as you lean forward. As you do, out slips an embarrassing fart that brings your cock to attention. You quickly undo the front of your pants and find your cock leaking pre already. With hardly a dirty thought in mind, you could feel an orgasm getting closer.
Your ears start to burn as they relocate higher up on your head, getting longer and more pointed in the process. It was as you were about to cum that you belch again, this time causing your body to prickle all over with goosebumps. It was a sensation that was hard to describe but you clenched your fists and went with it as you stained the front of your jeans with cum.
It was as this was happening that the strangest ache at the base of your spine took all of your attention. You crane your head around and look down to see a bulge at the back of your jeans. for the first few moments, you can't imagine what it could be until it gets long enough to really start straining your jean's fabric. Then, you realized you were growing a tail.
"Ah, ah, ah!" you yelp just as the back of your pants rips and two feet of ropy rodent tail flops from your back end. You look around confused and notice something on the ground near you. It's a rather fat looking rat, and it doesn't appear to be afraid of you.
Your attention is diverted to a pain in your taint behind your balls where your cock has started to shrink along with your testicles. Shoving your hand into your wet pants, you find your now tiny cock being shriveled away while a slit has opened up and swallowed what were your testicles. Gray hairs were sprouting around it and causing you to start to worry for your gender as you grew A cups on your chest.
Perhaps your hair would have gotten longer had it instead not started falling out entirely around you. Your nose started to hurt and now it was becoming all too obvious: you were becoming the rat around here and a female one at that.
The dizziness that followed made it hard to stand and hard to think. Your clothing began to feel larger and more empty as your body failed to fill them anymore. You tried to yell about then, as more rat hair started to cover your body, but your lungs were so squeezed that hardly a noise could be heard. The rat continued to wait eagerly, sporting a stiffy of its own, for the female rat to finish changing.
You were not too far away from it then, the moment when your clothes became cavernous and any semblance of you disappeared into a female rat. Not long after scurrying out a pant leg, the male rat was on you and greeted you to the animal kingdom with potent fucking that removed any human sense you once had. Left a dirty rat in the end, you scurry away back through the hole while a carny comes in to take your clothes and burn them in a nearby bonfire.