This is the most common skin tumor found in horses worldwide with six distinct forms and any tumor of the skin should be suspect as a sarcoid. History and distribution of the lesions suggest that flies are an important part of the pathogenesis. Currently there seems to be a role involving bovine wart causing viruses (papilloviruses) but live virus has not been isolated from any of these tumors. This article discusses the different appearances, clinical course, treatments and prognosis of this condition.