In 1933, virologist Richard Edwin Shope investigated strange the strange phenomenon found on wild cottontail rabbits in Iowa. He discovered that the hornlike protusions were in fact keratinous carcinomas and identified the cause as a virus. The Cottontail Rabbit Papilloma Virus, also known as the Shope Papilloma Virus is usually spread by parasites like fleas and ticks.

In essence, it causes the growth of cancerous, horny warts in the skin under the rabbit's fur. These tumors are in most cases benign, though they turn malignant in a quarter of cases. The virus is mainly found in the midwestern USA. Incidentally, high levels of vitamin A seem to in inhibit carcinoma growth.

Oddly enough antlers in deer are apparently derived from bone cancer some time in their ancestry. >A 2019 study published in Science identified eight genes active in antler formation that are normally associated with bone cancer, particularly osteosarcoma. Additional tumor-suppressing and tumor-growth-inhibiting genes were determined to be responsible for regulating antler growth. This was taken to indicate that antler formation is more similar to a highly controlled form of cancer growth than to normal bone development. We may end up with jackalopes yet.