Canine transmissible venereal tumor

A canine transmissible venereal tumor (CTVT), also known as a transmissible venereal tumor (TVT), canine transmissible venereal sarcoma (CTVS), sticker tumor or infectious sarcoma, is a histiocytic tumor of the external genitalia of domestic dogs and other canines, and is transmitted from animal to animal during mating. It is one of only three known transmissible cancers in mammals; the others are devil facial tumor disease, a cancer which occurs in Tasmanian devils, and contagious reticulum cell sarcoma of the Syrian hamster.

Source: Wikipedia — Canine transmissible venereal tumor (CC BY-SA 4.0)