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, but she was wrong because Finn came out "as pure as the driven snow." She is very good at hiding and camouflage since she has the same color as trees and wooden floors.
Thieves! It's the City of Thieves. Heed my warnings, boys. All who enter the city are destined to become thieves themselves. (cackles) Even you!
(cackle, cackle) Ooh, what did I tell you, boy? The city will get to you, no matter how pure you think you are! (cackle, cackle) I was right.
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.