Mast cell tumors are an uncommon occurrence in horses . They usually occur as benign, solitary masses on the skin of the head, neck, trunk, and legs. Mineralization of the tumor is common.  In pigs and cattle , mast cell tumors are rare. They tend to be solitary and benign in pigs and multiple and malignant in cattle.  Mast cell tumors are found in the skin of cattle most commonly, but these may be metastases from tumors of the viscera .  Other sites in cattle include the spleen, muscle, gastrointestinal tract, omentum , and uterus.