Chickens are animals found on Earth.
Crypto first meets the chickens at Turnipseed Farm. They are a neutral species as they appear to only to wander around the farm. Unlike the cows, they don't play any real sort of role in the beginning of the game's story and are just there to do whatever.
There is one notable chicken named "Joe Bob" who is mentioned by a cowboy's thoughts in Rockwell. He is said to be very lovable.