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 Coin Fish. They are harmless, as bumping into them does not hurt Mario.

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.