In Sandy Petersen’s Cthulhu Mythos for 5e, there are rules for PC cats.
In Tales of the Old Margreve, there is a sorcerer that curses the Griffon Queen of the forest and turns her into a cat.
The adventure itself doesn’t resolve what happens to the sorcerer.
I kind of want to run an adventure where the sorcerer shows up and turns them into a cat, and they have to get their curse removed.
I’m still thinking through if this is a great idea, or a terrible idea.