What could a lump at the top of your buttocks be?

Genetic remnants of a tail. While most humans aren't born with a tail, some actually have a small tail when they're born and it is usually removed before the baby is taken home.

Answer

The coccyx is the scientific name for the tail bone. The tail bone sits right at the top of the "butt crack." The coccyx is made up of a few small segments. Through an early injury or a malformation, the coccyx can present as a lump.

Answer

It could be a pilonidal cyst.