From the Super Mario Wiki
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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
Super Mario Sunshine
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. Jellyfish hesitate before moving in a set direction. 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.