Of all living things on earth, man's ass is unique.
Even human-related species, such as chimpanzees, bonobos, and gorillas, don't have human-like butts.
The human buttocks consist of a layer of external skin and subcutaneous fat superimposed on the left and right gluteus maximus and gluteus medius, which are attached to the pelvis, and the two muscles of the gluteus maximus are the largest muscles in the human body.
But for other mammals that have hips, the main component of the buttocks is the gluteus medius. For most mammals, large gluteus medius muscles are relatively normal.
This is actually a by-product of human walking upright, and it is also one of the milestones in the history of human evolution.
