A long appendage protruding from the lower back. Often covered in fur or scales. A common feature of animal girls.
In general, don't tag this on an animal unless there's something special or noteworthy about the tail, such as one of the many tags below that implicate this tag like heart tail or holding with tail, or if the image is focused on the tail. Definitely don't tag an animal with a tail of its own type, like tagging cat tail on a cat. (Do tag weird combos like a rabbit tail on a cat, though.)
The following tags implicate this tag: alpaca_tail, bear_tail, beaver_tail, bird_tail, cat_tail, chameleon_tail, cow_tail, crocodilian_tail, deer_tail, demon_tail, dinosaur_tail, dog_tail, dragon_tail, fake_tail, fiery_tail, fish_tail, forked_tail, fox_tail, giraffe_tail, heart_tail, holding_another's_tail, holding_own_tail, horse_tail, hugging_tail, jaguar_tail, laevatein_(tail), large_tail, leopard_tail, lion_tail, lizard_tail, long_tail, mechanical_tail, monkey_tail, moose_tail, mouse_tail, multicolored_tail, multiple_tails, otter_tail, penguin_tail, pig_tail, pokemon_tail, prehensile_tail, rabbit_tail, raccoon_tail, scorpion_tail, sheep_tail, snake_head_tail, snake_tail, spiked_tail, squirrel_tail, striped_tail, tail_between_legs, tail_biting, tail_fondling, tail_grab, tail_masturbation, tail_ornament, tail_rape, tail_through_clothes, tail_wagging, tail_wrap, tapir_tail, tiger_tail, weasel_tail, and wolf_tail (learn more).