Apparently 1 in 3 human babies is born with a substantial tail. It is just that humans are so repulsed by this that they remove the tail at birth. But Tail’s parents were different. They decided to keep the tail.
As a baby the tail was hidden by baggy diapers. When he was toilet trained the tail was wrapped around to the front and it looked like Tail always had a big belly.
It was always Tail’s decision whether or not to keep the tail when he was eighteen and old enough to decide for himself. He decided to keep it. He could easily lift half a ton with it and it made him able to swing through the forest like a monkey. He decided to keep it a secret and become a superhero with his rare abilities.
Mother Earth found an exoskeleton by one of Earth’s superpowers and adapted it for use with a tail. As a result, Tail is stronger and has more endurance than he would normally have.