A histiocytoma is a benign skin tumor found in dogs. The tumor can develop quickly. These growths are made up of rapidly developing cells and occur in dogs under 5 years of age. They vary in size but are usually raised and furless. They are most commonly found on the head, neck, ears, and limbs. Here is an example:   Diagnosis is made through cytology. A sample of the growth is collected by a few quick pokes with a small needle. This is then put on a slide, stained and examined through a microscope. This is what the cells look like: Most histiocytomas will go away on their own within two or three months. Surgical removal may be necessary if the tumor does not regress or if it grows rapidly to a large size. It is always a good idea to have your veterinarian check out any new lump that is found on your pet.