Shellcreeper

Shellcreepers, also known as Turtles and known as Kame-Sans (or Mr. Turtles) in Japan), 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.

Trivia

 * In the remakes of Mario Bros., Spinies replaced Sheellcreepers.