Two Headed Duck

The Two-Headed Goose is a two-headed goose that appears in the Adventure Time Theme Song. It is probably a mutated goose. The goose's two heads may be a result of the natural disaster in the Land of Ooo (Pendleton Ward said that the Land of Ooo is a post-apocalyptic world, the result of a horrible nuclear or natural disaster. The theme song provides evidence that the disaster was nuclear, as in the beginning, nuclear bombs are shown).

It can also be seen in the episode The Witch's Garden outside of the Tree Fort. Because the two-headed goose can only be seen around the Tree Fort, it can be assumed that it shares the house with Finn and Jake, or has a nest somewhere nearby.