Shellcreeper

Shellcreeper (Japanese シェルクリーパー / カメさん Shellcreeper / Kame-San) is a green-shelled turtle that first appeared in the Mario Bros. arcade game.

Mario Bros.
The Shellcreeper is the first enemy to appear in the game. They walk out of the Warp Pipes at the top of the screen and walk towards the pipes at the bottom of the screen. The player must hit it from below to flip it over and run into it to defeat it. If the player does not run into it, the Shellcreeper jumps out of it's shell (revealing that it wears a white vest and shorts), flips it's shell over and continues moving faster than before. Unlike the similar Koopa Troopa, jumping on Shellcreeper will cause Mario or Luigi to lose a life. Following the Mario Bros. style minigame in Super Mario Bros. 3, Shellcreeper has been replaced by Spiny in all subsequent versions, so they would not be mistaken for a Koopa Troopa, which is safe to jump on. Despite this, a lone Shellcreeper still makes a cameo alongside a Sidestepper and Fighter Fly on the GBA Mario Bros. title screen, so perhaps they are not totally forgotten.

Mario Power Tennis
Shellcreepers, along with the other Mario Bros. enemies, appear in the background of the Mario Classic Court.

Super Smash Bros. Brawl
Shellcreepers appear on the Mario Bros. stage. They walk out from the pipes at the top of the stage. If knocked over by hitting them from below or from an attack, they can be picked up and thrown at opponents, making them fly sideways off the screen.

Inspiration for Koopa Troopa
Shellcreeper is the first turtle enemy to appear in a Mario game, making it a conceptual ancestor of the Koopa Troopa that appeared 2 years later in Super Mario Bros. It is also shown that Shellcreepers wear vests and shorts when outside of their shells. Koopa Troopas first appeared out of their shells in Super Mario World wearing a vest, and in Super Mario World 2: Yoshi's Island wearing shorts.