It would be safe to say that sometimes DNA gets jumbled up wrong and it results in mutants. It happens for humans, as well as animals.
In this video, some of the world's abnormally large animals are displayed and elaborated. It's incredible that some of these animals are as big as they are; some of them even break world records.
They, being as big as they are, also eat a lot.
Whether it's a house pet, an outdoor pet, or a wild animal, mutations can happen anywhere. These miracles of life are rare and very intriguing.
Unfortunately, mutant animals typically don't live as long as normal ones.