Jelectro

Jelectros are glowing, electric jellyfish that stay in place underwater in Super Mario Bros. 3. They are invincible to all attacks; even Invincible Mario cannot defeat a Jelectro.

Super Mario Bros. 3
Jelectros appear in World 3-5 and World 7-4 of Super Mario Bros. 3. A large swarm in the middle of World 7-4 makes a hard time for swimming, especially since the screen moves automatically. Mario should try and swim slowly when surrounded by them.

The Adventures of Super Mario Bros. 3
Jelectros also make appearances in The Adventures of Super Mario Bros. 3. Here they are similar to their game appearances, except they have glowing red eyes and a more squared shape. They appear in episodes such as "Reptiles in the Rose Garden", where they can actually move, and "The Ugly Mermaid", where they stand still like they originally do in Super Mario Bros. 3.

Nintendo Adventure Book
In the Nintendo Adventure Book Leaping Lizards, Luigi spots some Jelectros through a window while exploring Iggy Koopa's secret underwater lab in Water Land. Three weeks later, some Jelectros appear in the swimming portion of the International Mushroom Games obstacle course. If the reader chooses Luigi to compete in the course, one of the Jelectros will shock and briefly stun him as soon as he enters the water.

Super Mario Maker
Jelectros are one of the course elements that can be placed in Super Mario Maker. They appear in the place of Spikes when creating a course in the Super Mario Bros. 3 Underwater style, and are only referred to as "Spike Traps".