Cecil says:

… something called canine transmissible venereal tumor, a very rare example of what’s known as a parasitic cancer. Unlike most other contagious cancers such as cervical cancer in humans, CTVT isn’t spread by a virus but (as recently proved) by cancerous cells themselves. Genetic analysis suggests the tumor originated in an individual wolf or domesticated dog, probably in east Asia, between 200 and 2,500 years ago. This long-dead canid’s much-mutated cells are still alive and being passed along during coitus (or sometimes through casual contact) centuries later, making it the longest-lived mammalian cell line known. The disease is now found throughout the world, especially where there are large populations of strays. It can be treated with surgery, radiation, and chemo, but most otherwise healthy dogs recover spontaneously after several months.

Luckily for us, there are no known parasitic cancers in humans, and only two additional ones affect animals. One arose spontaneously in a laboratory hamster around 1960: it’s a reticulum-cell sarcoma that can be spread by casual contact, cannibalism (hamsters’ souls are a lot darker than you might think), and mosquito bites; the tumor grows in the larynx and eventually leads to suffocation. The other is a condition threatening Tasmanian devils with extinction in the wild, called devil facial-tumor disease. First noted in the 90s, it’s spread by bites; the tumors grow around the mouth and eventually cause death by starvation.