The most likely and actual diagnosis in this case is fat necrosis. What is unusual is the absence of any trauma history or importantly any buttock injection. Nonetheless the mass has spiculated margins and seems to draw in the adjacent gluteus maximus muscle consistent with a fibrotic process. The buttock is an uncommon site for this pathology being more commonly seen in the breast.

The major differential diagnosis in this case is desmoid tumor (a.k.a. aggressive fibromatosis). It arises from fibroblasts that have undergone genetic mutation (in a gene called beta catenin). They can arise in virtually any part of the body and occur mostly in women in their 30's, but can occur in anyone at any age. They tend to be slow growing but can be extremely aggressive. They do not usually metastasize. Rarely they are associated with familial (colonic) polyposis syndrome i.e. Gardner's Syndrome.