Sarcoids, most common in Quarter Horses and related breeds, can occur anywhere on the body as flat ulcers, small nodules or large fleshy masses. Squamous cell carcinomas can also be ulcerative or fleshy growths, but they’re likely to affect white or pink areas of skin. Melanomas — another of the three most common equine skin tumors — appear almost exclusively, but not only, on older gray horses, usually around the eyelids, under the tail or throat, or in the perianal or genital areas. They may be black masses or just raised lumps under the skin.