Jellyfish

Jellyfish are water-dwelling, usually electrically charged creatures that made several appearances as enemies in Mario-related media. Their appearance is slightly distinct in almost every game they appear in, but always retain the usual traits of real life jellyfish, such as tentacles and globe-shaped bells. Jellyfish also serve as basis for other creatures found in the Mario games, including Jelectros from Super Mario Bros. 3 and Jellybeams from New Super Mario Bros. Wii.

Mario & Wario
In Mario & Wario, Wario uses a jellyfish on LEVEL6: Pukupuku Kai to obscure Mario's view.

Super Mario Sunshine
Jellyfish make a small appearance in Super Mario Sunshine, where Nokis are seen riding them in the episode Red Coin Fish. They are harmless, as bumping into them does not hurt Mario.

Yoshi's Story
In Yoshi's Story, Jellyfish only appear in the level Lots O'Jelly Fish. They are impervious to all methods of defeat, and come in blue, red, and green. Blue Jellyfish simply move up and down. Red Jellyfish chase Yoshi. Green Jellyfish simply float in place, and don't appear until the third part of the stage. Red Jellyfish can sometimes have an item like a Fruit or a Heart Fruit, and will chase prey.

Super Mario Galaxy
In Super Mario Galaxy, Jellyfish appear in a few galaxies, in the Deep Dark Galaxy and the Bigmouth Galaxy. In this game, they are blue, and are very large. If Mario comes in contact with one, he will be shocked. They can only be defeated by hitting them two times with a Koopa Shell, and will give one Coin when defeated. After they are attacked once, they glow green, but don't act any more aggressively.

Super Mario Galaxy 2
In Super Mario Galaxy 2, a Jellyfish appears in the Cosmic Cove Galaxy. It behaves the same way as in the predecessor, but instead of giving a Coin when defeated, it will give ten Star Bits.

Donkey Kong Country: Tropical Freeze
In Donkey Kong Country: Tropical Freeze, Jellyfish appear as common enemies in the water levels of Sea Breeze Cove. Their motion path may vary from individual to individual; some simply swim in a straight line up and down, while other revolve around a certain spot. If the Kongs touch one, they get electrocuted and lose one life from their health meter. Jellyfish are almost invulnerable; however, they turn into an item if the Kongs perform a Kong POW nearby. These enemies glow softly and appear distinguishable in detail even in the dark, as seen in Amiss Abyss, a level which marks their first encounter with the Kongs in the game.