The Giant Turtle was a large turtle person that once lived in the Land of Ooo. The turtle appears in the "City of Thieves" as skeletal remains that hold the City of Thieves on top of its shell. It is unknown if the City of Thieves was there before the Giant Turtle died.
The Giant Turtle was a colossal animal that likely died before the events of Adventure Time. Currently, only part of its skull and leg can be seen outside of its dark-green shell. There are stairs on one side of its shell that lead to the City of Thieves on the turtle's back.