Jellions look like regular jellyfish, except they have big, cute, black, shiny round eyes and mouths with sharp teeth in which they use to consume people and make Jellion Clones. They can't sting. During the beginning of the epsoide after SpongeBob passes by a creepy cave, the Evil Alien Jellyfish Overlord eats a jellyfish and then makes a bunch more Jellions. One of the Jellions eats Patrick and makes a clone of him thus Jellion Patrick is born. All the rest of the Jellions are then captured by Jellion Patrick (SpongeBob caught one for himself still thinking it's a regular jellyfish, and thought it was a new breed, because they didn't sting) and they both give out the Jellions to the citizens of Bikini Bottom. Afterwards, the Jellions managed to take over most of the citizens except SpongeBob and Sandy who both learn that the Jellion's weakness is mayonnaise. They then fought through the Jellion army using mayonnaise and eventually took down the Overlord. The Jellions were all destroyed, never to be seen again.
The name "Jellion" may have come from the words "jellyfish" and "clone."
It is confirmed, that even though they are an exact copy of a jellyfish, they cannot sting.
The Jellions are a parody of the plants from Invasion of the Body Snatchers.