Shellcreeper
A Shellcreeper (also known as a Turtle) is a low-slung, green-shelled turtle that roams the sewers.
Mario Bros.
The Shellcreeper is the first enemy to appear in the game. They come out of the pipes (like most other enemies). Hit it from below to flip it over and then run into it to defeat it. If Mario or Luigi fail to run into it, the Shellcreeper jumps out of its shell (and reveals that it wears shorts), flips its shell over and then continues moving, this time faster than before.
Notes
The Shellcreeper is clearly a conceptual ancestor of the Koopa Troopa. Aside from both being turtles, Shellcreepers also wear shorts when outside of their shells - something the Koopa Troopas started doing in Super Mario World 2: Yoshi's Island (in Super Mario World, Koopa Troopas wore undershirts and shorts).
In later remakes of Mario Bros., Shellcreeper was replaced by Spiny, so gamers new to the Mario franchise wouldn't mistake them for Koopa Troopas and attempt to jump on the green turtles (which, of course, results in a loss of an extra life in this game).
Names
In the Japanese version of Mario Bros., the Shellcreeper is named Kame-San. "Kame" means "turtle", and "san" is somewhat of a Japanese equivalent to "mister".
