Does that ought to exist on top of the other new animal_* tags to cover things like the reversed configuration of anatomically-correct kangaroos, the almost hairless balls of rabbits (as drawn by e.g. Mizzyam) and lemon_testicles of cervines?

External balls are suprisingly uncommon in mammals, but I don't see why we couldn't add tags for that.

I'd just sort them in animal_balls and see what we have. :P