Koopa is a turtle-like enemy, who first appears in original Super Mario Bros. for NES. They first appeared in two colors: red and green. Also, they appeared in two variations: Koopa Troopas (normal) and Paratroopas (Koopas with wings on their shells). It one of the few enemies of Mario universe who cannot be killed just by jumping on them. If jumped on him once, it comes inside a shell. The second time the player jumps on the Koopa, the Koopa shell starts moving forward until it hits a wall and bounces back. It can be stopped after jumping on it again. If left shell alone, Koopa will come out of its shell again. If player jumps on Paratroopa, he transforms into normal Koopa Troopa. In Super Mario World was introduced two new color variations: yellow and blue.
In A Super Mario World
In Super Smash Flash
The Koopas appear in the first stage of the Adventure mode called Super Mario World along Winged Goombas but unlike the Mario games, its shell cannot be used as a weapon as one hit destroys the whole Koopa. All the Koopas in this level are red-shelled, despite that, they behave like their green-shelled counterparts who keep walking and don't turn around when they encounter an edge, instead they fall.
In Super Smash Flash 2
This article/section contains information of a feature that is not confirmed to appear on any game or similar media. This means that the subject of has yet to be confirmed by an official source for the final build for said material.
Koopas are confirmed to appear once in the Adventure Mode stage: Mushroom Kingdom. It is most likely the shells can be used as weapons in SSF2 as it was said on the DOJO!!!.