Goomdiver

A Goomdiver is a diving-helmet clad Goomba found only in Mario and Luigi: Superstar Saga. They only appear under the water.

Oddly enough, near-identical Goombas wearing diving helmets actually did appear earlier. In Super Mario Land 2: Six Golden Coins's last level in Turtle Zone (the level where Octopus is fought), several of these Goomdiver-like creatures are littered about. In this game, one hit can defeat them, and the only difference between them and normal Goombas is that Mario slightly bounces off of them (while he merely lands atop a Goomba, flattening it with no upward propulsion).