A tumorous growth involving the pigment cells of the skin - the melanocytes. It presents as a roundish, hard mass and is generally subcutaneous (covered by a layer of normal skin). This type of tumour may be relatively benign (limited to the skin only) or malign (spreads to internal organs). Grey horses of advancing age are extremely prone to the disease; tumours are most frequently found in the parotid region, around the anus, on the dock and on (or in) the sheath of geldings.