Jelectro

Jelectros are glowing, electric jellyfish that slightly bob in place underwater in Super Mario Bros. 3. They are invincible to all attacks, even Invincible Mario.

Super Mario Bros. 3
Jelectros appear in World 3-5 and World 7-4 of Super Mario Bros. 3. In World 3-5, they are spread throughout the level, and in World 7-4, a large swarm of them can be found in the middle of the autoscrolling level.

In the GBA version, Jelectros bob up and down in place, rather than staying completely still.

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.

Super Mario Bros. & Friends: When I Grow Up
In the coloring book game, Super Mario Bros. & Friends: When I Grow Up, the coloring page for Marine Biologist has a Jelectro swimming with other underwater enemies and Frog Mario. The description for the job is as follows:

Mario the marine biologist has just discovered some new life forms living at the bottom of the sea: Cheep-Cheeps, Jelectros, Bloobers, and Trouters, to name a few.

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 / Super Mario Maker for Nintendo 3DS
After a twenty-four year absence, Jelectros appear as one of the course elements that can be placed in Super Mario Maker, and later in Super Mario Maker for Nintendo 3DS. They replace the Spike Traps when creating a course in the Super Mario Bros. 3 Underwater style, keeping their properties from Super Mario Bros. 3.