Some animals have what is known as "indeterminate growth" which means as long as they live, they will continue to grow and their ultimate size is only determined by their life span. Certain species of fish exhibit this trait, as well as animals like lobsters, crocodiles and alligators, and even kangaroos. It's rare to find giant specimens because obviously the longer an animal lives, the more chance it has of being caught by a predator or killed in some other way. However, indeterminate growth is most rare among mammals. Most mammals size is determined by genetics at birth and the skeleton stops growing once they reach adulthood. There is a differnece between genetic giants (people and animals that are naturally larger than others because of inherited genetics) and pituitary giants (people or animals that are larger due to increased levels of growth hormone). Almost all known animals have the same part of the brain that determines body growth as humans, the pituitary gland. Abnormalities on or malfunction of this gland can result in increased production of growth hormone. That is how human giants are created. Its just as common in animals as it is in humans, which is to say not very common.