Lorraine is a chicken who appears in "BMO Noire." BMO questions her about Finn's missing sock. She lives in a small structure on one of the branches of the Tree Fort, although Finn and Jake did not know her name until BMO tells them at the end of the episode. She was in a relationship with BMO in the past and was seeing both Ronnie and Bebe until they died.
However, this could all be a part of BMO's imagination, as suggested in "BMO Noire."
Finn mentioned she ran away in "Blank-Eyed Girl."
Finn also mentions "our chicken Lorraine" in the episodes "I Am a Sword"
Lorraine looks like a regular chicken with messily-applied lipstick on.