Shellcreeper

Shellcreepers are green-shelled turtles that first appeared in the Mario Bros. arcade game. They are the first turtle enemies to appear in a Mario game, making them the conceptual ancestors of the Koopa Troopas, which appeared two years later in Super Mario Bros.. It is also shown that Shellcreepers wear undershirts and shorts when outside of their shells; Koopa Troopas first appeared out of their shells in Super Mario World wearing a shirt, and in Super Mario World 2: Yoshi's Island wearing shorts.

Mario Bros.
The Shellcreeper is the first enemy to appear in Mario Bros.. 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 its shell (revealing that it wears a white shirt and shorts), flips its shell over, changes color from green to purple and continues moving faster than before. If it flips its shell again, it will turn red, its fastest state. If a Shellcreeper is the last target for the phase, it will automatically turn red. 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Following the Mario Bros.-style minigame in Super Mario Bros. 3, Shellcreeper has been replaced by Spiny in all subsequent versions, so they would not be mistaken for a Koopa Troopa, which is safe to jump on.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 Game Boy Advance Mario Bros. title screen.

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

Super Smash Bros. Brawl
In 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. The Shellcreepers also have their own trophy.

Super Mario 3D World
Shellcreepers appear in Luigi Bros.; an additional game featured in Super Mario 3D World. Like the other enemies in Luigi Bros., Shellcreepers behave in the same way as Mario Bros., which this game is based on.

NES Remix
Shellcreepers appear in NES Remix in the challenges and remixes based on Mario Bros.

Trivia

 * The Shellcreeper and the manner in which it was defeated was indirectly alluded to in the final battle of Super Paper Mario, when Super Dimentio tells the party that the destruction will begin with them as they "wallow in helplessness like upside-down turtles!"