Deep Cheep

A Deep-Cheep is a special variety of Cheep-Cheep found in New Super Mario Bros. and New Super Mario Bros. Wii. Unlike regular Cheep-Cheeps, these creatures are green and will actively hunt Mario or Luigi down in the underwater levels. Deep-Cheeps will only give up the pursuit after the plumbers swim far enough away.

Deep-cheeps are incredibly annoying in large quantities, but if you are Fire Mario. you can kill them with fireballs.