Finn was, for a long time the only confirmed human in the Land of Ooo. The reason for this is revealed in the episode "Her Parents," as Lady Rainicorn's dad believed that humans were extinct.
Up until "Susan Strong," Finn believes he has never met another human and becomes morose and "soul-searchy" when he thinks about it. However, Finn encounters Susan Strong a member of a tribe of mutants called Hyoomans. However, in "Beautopia," it is hinted that Susan Strong is unlike the rest of her tribe and may potentially be another human. In "Dark Purple," Susan removes her hat and reveals she is a human with a device on her head, making her a cyborg.
In "Billy's Bucket List," Finn is told by a deceased Billy that his father, Martin, is still alive and is trapped in the Crystal Citadel. This prompts Finn to go there to free his father in "Escape from the Citadel," making him one of four known humans in existence.
Although various other humanoids exist in the Land of Ooo, most have been confirmed by the show's creator, Pendleton Ward, to be mutants, such as Penny, Phil or the Old Man Henchman.[citation needed] According to Pendleton Ward's FAQ page, even Finn is mutated in some way.[2]