We humans are a bit of an oddity in lacking a tail. It’s only really the apes, among vertebrates, that have dispensed with a posterior extension of the spinal column. All that remains of ours is the coccyx, the only function of which is apparently to hurt very much when someone pulls a chair from beneath us as we go to sit down.
Why do animals have tails?
Other vertebrates make good use of their tails. In fish and marine mammals, tails are primarily a means of propulsion through the water. They are involved in locomotion on land, too. Lizards that have lost theirs to predators are slower than intact animals because tail movements help drive the legs.
In cats, tails are crucial for balance during acrobatic movements. And many monkeys and other climbing animals use theirs as a fifth grasping limb. Horses and cattle swish their tails to deter biting flies. Porcupines and pangolins wield them as defensive weapons. Among dogs and many birds, tails are central to visual communication.
Why don't humans have tails?
It’s not known why we apes lost our tails, though it may be connected with a change in locomotion style when our ancestors descended from the trees.