Jellybeam

Jellybeams are a type of 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 slightly reminiscent of Jelectro 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.

New Super Mario Bros. U
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, Fish Bones and Circling Boo Buddies.

Despite helping Mario to see underwater, they can also hurt him if Mario touches them. Jellybeams can be defeated with fireballs or a Star. Mario can also freeze them, causing them to stop moving, but they will continue to illuminate the water. Their movements can be compared to a Thwomp, although rather than slamming down on the ground when Mario approaches, they automatically move up and down slowly.

Mario Kart 8 / Mario Kart 8 Deluxe
Jellybeams are also seen in the underwater sections of the GCN Sherbet Land and Dolphin Shoals tracks in Mario Kart 8 and Mario Kart 8 Deluxe.