The Dragon Jellyfish is a huge jellyfish with pink scales on his head, six long tentacle stingers, a barb, eight red polka dots (just like the regular jellyfish), and a dragon tail. He is very powerful and has the ability to burn others. He is also very fond of Krabby Patties. He was owned by Planktonamor, until he stung him after tasting SpongeBob's Krabby Patties and switching sides.
The Dragon Jellyfish hasn't been seen since "Dunces and Dragons," as SpongeBob will probably never go to medieval times again.
The closest jellyfish in relative size to the Jellyfish Dragon was the King Jellyfish.
Just like his possible descendant, King Jellyfish, Dragon Jellyfish seems to be easily calmed down when given food, as he stopped his rampage after being given a Krabby Patty by SpongeBob, and the King Jellyfish stopped his rampage after SpongeBob gave his a bubble resembling a delicious pie.
It is possible that Dragon Jellyfish is who King Jellyish gets the trait from.