Beach Koopa

A Beach Koopa, originally known as Koopa without a Shell, is a Koopa Troopa that has lost its shell and wears nothing more than an undershirt (and/or, in some cases, boxer shorts), first appearing in Super Mario World. In their original appearance, Beach Koopas appear small and hunched over, but were later depicted as being able to walk around like normal, shell-wearing Koopa Troopas. In the Mario platformers, when a Beach Koopa finds a shell, it reverts to a normal Koopa Troopa.

Mario Bros.
While technically not shell-less Koopas, in the original arcade version of Mario Bros., if the Shellcreepers are left alone, not attacked, while they are upside down, they will pop out from their shells to flip back their shells. When this happens, they highly resemble shell-less Koopas, wearing a sleeveless white shirt and shorts.

Super Mario World
When first knocked out of its shell, the Koopa is extremely vulnerable. If given enough time, it will right itself and pace forwards in search of a new shell. They act very similarly to their shelled counterparts. Green ones fall off cliffs, while Red ones and the slightly faster Blue ones try to avoid falling; the Yellow ones act like a quicker Green one, but will also follow Mario or Luigi. The technical reason for the small, hunched-over appearance is so the "sliding out" graphics have the same hitbox parameters as the walking sprites, as each sprite for each individual object in Super Mario World must use the same hitbox.

Green and Red Koopas without Shells will become the color of the shell they enter. If the shell is upside-down, they will kick it right-up before entering. Yellow Koopas without Shells will attempt to jump into any nearby shell; this also allows them to dodge shells thrown at them. Once a Yellow Koopa without a Shell enters a shell, it will become a flashing Shell, which will flash rainbow colors and spin around after Mario. If two Koopas without Shells enter the same shell simultaneously, they will become only one Koopa Troopa.

Blue Koopas without Shells are visually distinctive, with a larger body and lack of vulnerability upon being removed from their shells. Instead of re-entering them, they will kick any shell they come into contact with, even stopping moving ones thrown at them; they will also do the same with stunned Goombas, Bob-ombs, flashing Grab Blocks, and Buzzy Beetle shells in their path. Blue Koopas without Shells may also appear sliding down slopes towards Mario. Any Super Koopa who wields a flashing cape will revert into a Blue Koopa without a Shell after being stomped on and relinquishing its Cape Feather.

The original artwork of a Koopa being knocked out of its shell serves as the basis for a similar art work that appears as a stamp in Super Mario 3D World.

Super Mario 64 / Super Mario 64 DS
In Super Mario 64 and Super Mario 64 DS, Mario or others can attack a Koopa Troopa to knock its shell away, transforming the Koopa into a Beach Koopa. If attacked again, the Beach Koopa would be destroyed and a Blue Coin would pop out. If not defeated, the Beach Koopa will frantically attempt to dive back into its shell even if Mario is riding it, sometimes killing itself. If the Beach Koopa is seen again after the shell is destroyed, it will still act as if it never lost its shell.

Super Mario Sunshine
In Super Mario Sunshine, Blue Electrokoopas throw their shell at Mario and effectively become Beach Koopas temporarily. If Mario manages to jump on it or squirt it with water, so it is electrified by its own shell, it will be defeated. These Koopas have pink boxer shorts.

Super Mario 3D World / Super Mario 3D World + Bowser's Fury
Beach Koopas reappear in Super Mario 3D World and its Nintendo Switch port. Unlike in other games, they share the same behavior as regular Koopa Troopas in the game, except they have an angry look. If the player keeps them away from their shells long enough or throws them away, they will become disappointed and briefly stop chasing the player.

Super Mario Maker / Super Mario Maker for Nintendo 3DS
Beach Koopas reappear in Super Mario Maker and Super Mario Maker for Nintendo 3DS, only in the Super Mario World style when a Koopa Troopa is jumped on.

Super Mario Maker series
In Super Mario Maker, Beach Koopas appear after they are knocked out of their shells in the Super Mario World game style. Red Beach Koopas make their first appearance since Super Mario World, only appearing if a Red Koopa Troopa in the level is stomped on.

In Super Mario Maker 2, the above is also true of the Super Mario 3D World game style; Red Beach Koopas make their first 3D appearance, and Beach Koopas can also drive Koopa Troopa Cars in the Super Mario 3D World style. After defeating them, the player can then drive the car themselves.

Super Mario World television series
Only appearing in the Super Mario World episode "Fire Sale", the silent, Koopa Troopa henchman of Kootie Pie Koopa is continually knocked out of his shell, becoming a Koopa without a Shell, in the episode, usually during one of Kootie Pie's violent temper-tantrums. It wears yellow shorts with red hearts on them.

Mario is Missing!
Although normal Koopas without Shells do not appear in the MS-DOS version of Mario is Missing!, the ending features Luigi removing Bowser's shell. When he does so, Bowser is shown to be wearing polka-dotted boxer shorts, similar to a Koopa without a Shell. In the NES version, a normal Koopa without a Shell appears after the small Bowser's defeat.

Yoshi series
Beach Koopas are also found in Super Mario World 2: Yoshi's Island, Yoshi's Island: Super Mario Advance 3 and Yoshi's Island DS, although here, they walk upright when evicted from their shells, instead of becoming small and hunched over, as in Super Mario World. This is due to them not ever sliding, meaning they can have the same hitbox as an ordinary Koopa Troopa. In these games, they are shirtless and wear colored shorts, likely inspiring their name. Additionally, in Yoshi's Island, Hookbill the Koopa becomes a Beach Koopa upon defeat.

Paper Mario
Beach Koopas make a brief appearance in Paper Mario, when the Fuzzies appearing in Koopa Village stole the local Koopa Troopas' shells. This fate also befalls Kooper, though Mario eventually returns his shell to him.

Super Princess Peach
Beach Koopas reappear in Super Princess Peach, appearing as they do in the Yoshi sidescrollers. Like Blindfold Boos, blue Podoboos, and Security Thwomps, they do not appear in the in-game glossary.

Mario & Luigi: Paper Jam
Beach Koopas also appear in Mario & Luigi: Paper Jam. If the player counterattacks a Koopa Troopa's shell attack with the hammer, it will get knocked out of its shell before it gets back in.

Other appearances
The concept of a shell-less turtle enemy was planned for the original Super Mario Bros. It had the tentative name 「ベア クリーパー」 (Bea Kurīpā, Bare Creeper), and it appears to have held a pointed weapon.