Jelectro

Jelectros are glowing, electric jellyfish that stay in one place underwater in Super Mario Bros. 3. They cannot be defeated by any means and will hurt Mario if touched. A large swarm in the middle of level 7-4 makes a hard time for swimming, especially since the screen moves automatically. Mario should try and swim slowly when surrounded by them.

Two enemies from Super Mario RPG: Legend of the Seven Stars (Muckle and Leuko) look extremely similar to Jelectro.

Television
Jelectros also made appearances in The Adventures of Super Mario Bros. 3. Here they were similar to Bloopers in appearance, except they were yellow with blue near their red (or bloodshot) eyes. They appeared in episodes such as "Reptiles in the Rose Garden". Here they were aggressive and would zap Frog Mario. However, they didn't appear in many episodes.