Certain bones or organs, scales, tissues - in some animals of human beings are not used. They remain in them because they inherit the genes from their ancestors and so those organs are present from the time of birth.
These are called vestigial structures or features.
It happens due to natural evolution over centuries. Certain organs are not used due to changed lifestyles. They slowly diminish and vanish in the process of evolution of species.
In human beings it is said that the tail bone is not used. Some bones in humans are underdeveloped as compared to animals. So we cannot hear as good as them. A whale has very small hind legs that are not used.
There are some more examples related to cats, fish insects and so on.