This tumor is likely a melanoma, which is common in grey horses over four years of age. Although most are benign, some are cancerous (metastatic and invasive) in horses, whereas in people true melanoma is cancerous. Most of these tumors are slow growing, and treatment may vary with the size of the mass, the age of the horse, the use of the horse, and economic considerations. Surgical removal is often successful in reducing the mass, if not eliminating it. Laser surgery is often used as it is less invasive and more successful than traditional surgery; however, it may not be the treatment of choice if the mass(es) are large. Since these tumors usually grow slowly, many horses receive no treatment and go on for years without problem other than cosmetic concerns. If the tumors ulcerate, they may attract flies, and some tumors may become large enough locally to occlude the anus (preventing defecation) or vulva (preventing foaling).

To determine the best course for your gelding, you should contact your veterinarian to determine the appropriate diagnosis and treatment.