Poisonous Jellyfish

 are a specie of jellyfish introduced in "Whipple the Happy Dragon" when they attach to Jake's stomach while he has shape shifted into a boat. They also attached to Susan's head.

Abilities
They have the ability to make the thing it is attached to have hallucinations as seen when Jake has a hallucination of Joshua and Margaret and when Susan has a hallucination of Frieda.