Chicken

Chickens (Gallus gallus domesticus) is a domesticated fowl, a subspecies of the Red Junglefowl, and one of the most common and widespread domestic animals in the world.

Background
Although no physical chickens appear in any Fallout game, they are mentioned in the present tense. As they are a staple of the human diet. For example Modoc, where Rose passes her famous Wasteland omelet as chicken egg. Even going so far as to only refer to her "Chicken" as a chicken and its cage as a "chicken coop". Although this doesn't really fool the more attentive locals, not because chickens are nonexistent but because of her heavily fortified "chicken coop".

As a staple chicken is a widely known flavor, including but not limited to Vault dwellers, and tribals.

On Mount Desert Island there exists a population of "rad chickens" that closely resemble pre-War chickens with little mutation.

Biology
Although some chicken may be mutated, the majority are not.

Appearances
Actual chickens appear in the Fallout 4 add-on Far Harbor, as well as Fallout 76. They are also referred to in Fallout 3, sarcastically in Fallout, Fallout 2 as taste and Fallout Tactics in the form of Graffiti. It is also a term referring someone as being afraid and/or a coward.