The real answer is while some animals use there tails for balance, not all do. Some animals have tails so they can use them to hang from trees serving as an extra hand. Cows, zebras, an Elephans have tails to swat at annoying flys. Fish have tails for swimming. A lizard has a tail for defense.
Horses use them to swat flies. Dogs use them to signal happiness, excitement and anticipation. Some deer have black and white tails which make a good, visible signal for their youngsters to follow when they run.