Long furry (or scaly) appendage protruding from the lower back, a common feature of cat girls. In Japanese: Shippo.
In general, don't tag this on an animal unless there's something special or noteworthy about the the tail, such as one of the many tags below that implicate this tag like heart tail or tail hold, or if there's an image focus on the tail. Definitely don't tag an animal with a tail of its own type, like tagging cat tail on a cat. That's assumed. (Do tag weird combos like a bunny tail on a cat, though.)
source: danbooru wiki