you had found this book in your attic, it had a bunch for spells, one of them was a "ideal world" or "ideal" dimension spell. You could create a dimension for whatever you want, so you created a portal to a petspace dimension, which will keep you safe and in pet space for as long as you want, but the error is you put as long as possible, so entering the dimension you didn't realize it'd try to trap you in pet space, you enter, and are given the ears, and tail, of your choice, and you get a nice collar on your neck (optional)