What Type of Cyst or Tumor Is This?

It isn't possible to identify a lump reliably from photos; even specialists need cells or tissue to know.

A good rule from veterinary oncologists is "see something, do something": any lump the size of a pea (1 cm) or larger that has been there for a month should have a needle aspirate or biopsy. Your vet can do a needle aspirate in a few minutes.

See lumps and bumps on dogs.

This is general information, not a diagnosis. Please follow your own vet's advice.