Poisonous Jellyfish are a species of jellyfish introduced in "Whipple the Happy Dragon" that live in Poison Reef. The jellyfish attach to Jake's abdomen when he was shape shifted into a boat, and attached to Susan's head causing both of them to hallucinate.
They have a bright, bluish-purple bell with a purple brim around it. It's tentacles are the same color as the brim of the brim of the bell and are curly and twisty.
Poisonous Jellyfish have the ability to excrete hallucinogens onto their victims causing their prey to experience vivid hallucinations. This was demonstrated when Jake was hallucinating and saw Joshua and Margaret, and when Susan saw Frieda, despite having no memory of her at that time.