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.

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".