Animal with long tail

i presume you mean, Why do animals have long tails? Mainly for balence, which is why so many monkeys and tree dwellers have extra long ones. Some, but i doubt many may use it as a defence system like dinosaurs would have. it may also have some sort of impact on what your rank is in a group of social animals, again such as monkeys. The length of the tail may also have some impact on mating and finding a mate.