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. The trophy descriptions in Super Smash Bros. for Wii U state that the Koopas apparently evolved from Shellcreepers; however, the Japanese text specifies Koopa Troopas.

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Following the Mario Bros.-style battle mode in Super Mario Bros. 3, Shellcreepers has been replaced by Spinies in all subsequent versions, in order to make sure that players who played the Super Mario series won't mistake them for Koopa Troopas, which are safe to jump on. Despite this, a lone Shellcreeper still makes a cameo alongside a Sidestepper and Fighter Fly on the title screen of the Game Boy Advance remakes of Mario Bros.

Mario Bros. Special
In Hudson Soft's 1984 sequel of sorts to Mario Bros., Mario Bros. Special, Shellcreepers make a return appearance as enemies, though this time they can't be stunned by hitting them from below; instead, they can be stunned by jumping on a trampoline that they're on (second stage), jumping on a conveyor belt that they're on (third stage), or killed instantly by jumping over or next to them and hitting a platform, though they will often respawn if killed this way.

Punch Ball Mario Bros.
Shellcreepers appear as an enemy yet again in Hudson Soft's semi sequel to Mario Bros., Punch Ball Mario Bros.. The game is much closer to the Mario Bros., and thus Shellcreepers play a much more similar role as they did in that game. The only differences are that they can't be stunned from below and are instead stunned by Punch Balls, and that they do not change color if they get back up after being stunned.

Mario Power Tennis
Shellcreepers, along with the other Mario Bros. enemies, appear in the background of the Mario Classic Court in Mario Power Tennis, where they use their NES sprites. The game's data includes an unused 3D depiction of a Shellcreeper, showing it as a small, quadrupedal Koopa Troopa with clawed flat feet.

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.

Super Mario Maker
In Super Mario Maker, a green-colored Shellcreeper is present as an unlockable playable costume. Its look is based on that from Mario Bros., and its movements as well. Whilst the player presses on the GamePad, the Shellcreeper shows off its teeth in a menacing yet comical way; likewise, if the player dashes with the character, its shell will turn red.

Names in other languages
Schildkröte