Jellybeam

Jellybeams are a jellyfish enemy introduced in New Super Mario Bros. Wii. Their name is a pun on, as well as a portmanteau of "jellyfish" and "beam". Their appearance is reminiscent of Jelectros from Super Mario Bros. 3.

New Super Mario Bros. Wii
In New Super Mario Bros. Wii, Jellybeams are only found in World 8-4 and serve to illuminate the dark waters. They are found along with Stone Spikes, Cheep Cheeps, Deep-Cheeps, Bloopers, and Bulbers.

Jellybeams move up and down slowly. Despite helping Mario to see underwater, they can also hurt him if Mario touches them. Jellybeams can be defeated with fireballs or a Super Star. Mario can also freeze them, causing them to stop moving, but they will continue to illuminate the water.

New Super Mario Bros. U / New Super Mario Bros. U Deluxe
Jellybeams reappear in New Super Mario Bros. U acting in the same manner as in New Super Mario Bros. Wii, and appear only in Deepsea Ruins of Soda Jungle with Bulbers, Wet Bones and Circling Boo Buddies.

Mario Kart 8 / Mario Kart 8 Deluxe
Jellybeams appear in the underwater segments of GCN Sherbet Land and Dolphin Shoals in Mario Kart 8 and Mario Kart 8 Deluxe. They illuminate the water around them but do not actively interact with drivers.

The Super Mario Bros. Movie
An illustration of a Jellybeam can be seen on a storefront sign in the teaser poster for Illumination's The Super Mario Bros. Movie.