Hyoomans (referred to by Jake as Fish People) first appear in "Susan Strong." They are a race of primitive ichthyoid mutants who, like Finn, wear animal-themed hats. At one point, led by Susan Strong, they attempted to invade and devour the Candy Kingdom but were ultimately driven away through their fear of fire.
Despite their understandable mistrust after the attempted invasion of the Candy Kingdom, Finn and Jake help the Hyoomans by helping them reclaim their ancestral home of Beautopia in the episode of the same name.
The Hyoomans were originally discovered underground, in a watery cave, accessed by a hatch in a clearing in the candy kingdom. In the original storyboard of Susan Strong, the hatch had "Biohazard Danger" printed on it, alluding to the Hyoomans' mutated nature. The warning does not appear in the final cut of the episode.
In the episode "Beautopia," the Hyoomans' grotto is revealed to be a part of a much larger cave system home to a massive subterranean lake, beneath which lies an entire ruined city. Also within these caves is the mysterious Beautopia, a ring-shaped settlement surrounding a central lighthouse. Susan reveals that Beautopia was the home of the Hyoomans long ago before they were driven out by the shadowy Lub Glubs. At the end of "Beautopia," Finn and Jake relight the lighthouse, which destroys the Lub Glubs and allows the Hyoomans to resettle. Beautopia remains the Hyoomans' current residence.
Hyoomans appear mostly humanoid except for their distinctly ichthyoid heads. Although the particular fish-like features vary from Hyooman to Hyooman, they often include various fins and gills and are always localized to the area of the head surrounding the face – never on the face itself. Because of this, it can be difficult to tell fully clothed Hyoomans apart from ordinary humans, as their animal hats obscure their mutant countenance. Hyoomans have yet to be observed in an underwater environment, so whether their fish-like appendages are functional or simply vestigial has not been determined.
- Finn teaches the Hyoomans various words when he still thinks they are human; they become known as Hyooman after the way they mispronounce the word "human."
- The animal hats on the Hyoomans' heads at first identify them with Finn, who also wears an animal hat; however, the hats ultimately prove to have been hiding the Hyoomans' real appearances.
- In the episode "Everything Stays," Marceline encounters a tribe of post-war human survivors who all wear animal hats, very similar to those of the hyoomans. However, in "Min and Marty," it is shown that at least some of these survivors went on to be the founders of the human colonies in the islands. Given this, it is possible (though not confirmed) that the Hyoomans are descendants of some mutated human survivors that did not manage to sail away from Ooo.
- Many Hyoomans shown are able to speak in broken English. This may have been passed down from pre-war society, or the Hyoomans may have simply learned what little English they know from Susan.
- While she lives with the Hyoomans in their underground society, Susan Strong is, in fact, a normal, unmutated human. This is revealed in the episode "Dark Purple" when Susan rips off her hat to keep from suffocating.