Hyooman tribe

The Hyoomen tribe appears in the episode Susan Strong. They are a race of mutant humanoids who wear animal hats like Finn. They are very paranoid and talk in strange verses. After Susan discovers the Candy Kingdom, they threaten to eat all of the kingdom and the people in it as a means to sustain themselves. Nearly all Hyoomens (after taking off their animal themed hats) have what appear to be gills and fins on their heads, referring that the Hyoomens aren't human, but may be mutated. This is also supported by the fact that the hatch which leads down to the Hyoomen tribe's home has "Biohazard Danger" written on it, though this was removed in the final cut of the episode. Susan does not take off her hat and it is not revealed if she is a true human or not, leaving Finn in awe and surprise.

It may be possible that Susan may not be a true human and may just be a Hyoomen. The girl in the photo to the left named Celina was one of the Hyoomens, and just like Susan, she posseses hair coming out of her hat. When she removed her hat, fins were present.