An animal’s tail can serve the animal in many different ways. When this question is asked, you principally see explanations of it as a counterbalance measure for the animal. Arboreal species may require very specific balance when walking on thin tree-limbs. Even in a cat, where the tail is not at all times useful, its mechanism allows it to “land on its feet” if the cat falls.
Other arboreal species like the possum have tails that can actually help them hang onto trees and catch branches as needed. Many primates, like lemurs, also use it to enhance tree-climbing ability and to provide balance. Yet balance is not the only explanation.
In many new world monkeys, tails are prehensile. The prehensile tail allows the animal to snatch or grab with it. It can grab onto tree limbs, or even pull food off bushes. It really serves as an extra hand. Some animals have what is called a partially prehensile tail. It can’t be used for food gathering, but can be used for holding on to trees. Animals with partially prehensile tails include rats, tree porcupines, and anteaters, as well as reptiles like snakes and many species of lizards and newts.
A tail can serve completely different functions for an animal. Obviously a horse doesn’t need to snatch or grab at trees. Horses, and other farm animals like cows, use tails primarily to aid in their comfort. Their swishing action can help keep annoying flies from biting the animal. Many others animals have this fly swatter action, like the giraffe, zebra and elephant.
The tail and its feathers in birds can serve in many ways. First, in birds that fly, it may be used for direction and better aerodynamics when flying. Flightless birds, like penguins, use them when swimming to change directions. Tails in male birds often help to attract mates. Nowhere is this more evident than in the peacock, which has elaborate feathers meant to stir the interest of peahens.
In marine animals, tails are extremely important. Powerful thrusts of the dolphin's tail push it forward more quickly. In many fish species, it also promotes better and faster swimming and directional capabilities.
The tail in the lizard may be partially prehensile, but is also used as a defense mechanism. When lizards attempt to escape predators, many of them have tails that can safely detach. This allows the lizard to escape a predator that grabs it. Further, the lizard can grow a new one after it is detached, so it can live to defend itself another day.
There are some very specialized tails in the animal kingdom. The rattle at the end of the rattlesnake’s is a warning to predators to stay away. Deer use their tails to communicate potential danger. The powerful sting on the end of the scorpion’s tail makes for an excellent weapon, as does the long strong tail of the crocodile. Badgers have flat ones useful for swimming and conferring warnings, and dogs use theirs to communicate emotions. Specific benefits in each animal are often beautifully adapted to the animal’s needs.