The atherom is a firm cluster of sebaceous skin glands, basicly characterizing a cyst of the subcutanious tissue.
Sebaceous cysts appear as small lumps on the skin. They usually grow to the size of a cherry. The surrounding skin usually shows no further signs. The lump feels hard and is immovable. In some cases, sebasceous cysts may cause itching which might cause actions that lead to acral lick dermatitis.
Epidermal cysts develop because of an occlusion of an excretory duct for the sebaceous gland secretion.
Provided other types of a tumor can be excluded and the atheroma does not cause any disorders, it is left as it is. Otherwise, an atheroma needs to be removed surgically.
If you noticed a suspicious skin lump on your dog, please call your veterinarian at next opportunity and agree upon a date within the next days or week.