The Jellions

The Jellions are the main antagonists of the Spongebob Squarepants episode "Planet Of The Jellyfish". They came to replace all of the inhabitents of Bakini Bottom, but were defeated.

Biograph
Jellions look like regular Jellyfish, except they have big, 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 ruler eats a jellyfish and then makes a bunch more Jellions. One of the Jellions eats Patrick and makes a clone of him thusJellion 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 Squarepants and Sandy Cheeks who both learn that the Jellion's weakness is mayonnaise. After they fought through the Jellion army using mayonnaise and eventually took down the Overlord, the Jellions were all destroyed, never to be seen again.

The Evil Alien Jellyfish Overlord
The Evil Alien Jellyfish Overlord looks like a giant, alien jellyfish and had the ability to eat regular jellyfish and create Jellions in their place. It then tells the newly made Jellions to eat everyone in Bikini Bottom, turn them into Jellion Clones, and take away the real copies. Near the end of the episode, the Overlord somehow ends up in the Kraby Patty (probabilty flew/swam over there) where it held everyone, even SpongeBob's spatula, captive in pods inside the freezer. When SpongeBob squirted mayo on it, it was melted away along with all the other clones.