Hag makes her appearance in "City of Thieves." When Finn and Jake are in Stock Woods, she appears from a tree and warns them not to go to the City of Thieves. She appears again after Finn steals a chest from a tower to prove that she was right. After that, Jake kicks her away. She claims that anyone who enters the city becomes a thief. Her actual promise was that Finn would come out of the city a thief. She was wrong because Finn came out "as pure as the driven snow." She is very good at hiding and camouflage. She has the same color as trees and wooden floors.
According to Adventure Time character designer Phil Rynda, the Hag is a Tree Witch, like the character who appears in "To Cut A Woman's Hair." They possesses similar powers. They are also the only known characters of this species.