Radstag

Radstags, although "stag" once only was used to refer to a male deer, now it refers to the entire species of mutated deer.

Background
The radstag is a mutation of the stag species that once inhabited the forests of the Eastern Seaboard of the United States, having grown two heads like the brahmin, along with a pair of vestigial hoofed limbs protruding from its chest. Radstags typically avoid confrontation while grazing on the Commonwealth's grasses and shrubberies, preferring to flee any predators threatening them. However, when backed into a corner or otherwise forced to fight, it will stand its ground and use its horns to defend itself.

Many populations of radstags are fundamentally flawed and permanently damaged by the mutations they suffered. The Island, for example, is home to erratic and devolved radstag herds, which have lost all of their fur, with skin hardened by radiation and exposure to the fog into leather. The degenerative mutations actually increase their resilience and ferocity, despite gaping holes in their bodies and malformed horns.

Biology
 Descended from deer, the radstag is primarily a prey creature. It's quite fast and resilient, requiring a bit of precision when hunting. This creature drops radstag meat and a radstag hide on death. They are dicephalic, or two-headed; each head is able to swivel and turn on its own. A closer inspection of the heads shows that each head's inner eye is blind. Radstags also have two underdeveloped legs hanging limply from their chest, qualifying them as hexapedal. Aside from the ability to bend when a radstag lays down, these legs are nonfunctional.

Radstag are usually brown-furred and patchy in places, with small pus-filled cysts from radiation, which most often cover the flanks, feet, heads and snouts. The cysts more frequently appear on the rabid and glowing varieties and can cover most of the body. Radstag yearling coats are more colorful than those of adult radstag and feature white spotting, similar to the fawns of their deer ancestors. 

Appearances
Radstags appear in Fallout 4 its add-on Far Harbor and Fallout 76.