Koopa Troopa

Koopa Troopas are recurring enemies in the Mario series. They are turtle-like creatures who hide in their shells when someone stomps on its head. The Koopa Troopa is the most common classification of Koopa. Koopa Troopas are the standard troopers of the Koopa Army who work for Bowser, King of the Koopas. Koopa Troopas are all common Koopas, meaning they have no special abilities or characteristics. As the lowest ranking members of the Koopa Army, they just patrol their assigned areas on foot. Koopa Troopas normally come in two colors: Green and Red. If an enemy is referred to as just "Koopa" this usually means Koopa Troopa.

Mario Bros.
Koopa Troopas first appeared in the arcade game Mario Bros. under the name "Shellcreepers" (The Japanese version simply called them "Turtles") as enemies that have to be flipped. They started with green shells, and if not kicked off after being flipped over, would hop out of their shells and right themselves, turning red, then purple, and moving faster each time. (In remakes, Shellcreepers were made to look more like Spinys so that people would not try to jump on them.)

Super Mario Bros
In Super Mario Bros., they were renamed "Koopa Troopa", and Koopa Paratroopas, Koopa Troopas with wings, made their debut. They can be used as projectiles after they retreat into their shells. Green Koopas simply walked forwards, turning around if they encountered a wall or another enemy, and if they reached a ledge, would walk straight off, even into a bottomless pit. Red Koopas were mentioned in the manual as being more powerful than their green counterparts. In the game, this meant they had the sense not to walk off edges.

Super Mario Bros. 3
In Super Mario Bros. 3, Koopa Troopas stayed the same as in Super Mario Bros. One thing that changed was, when the return to their shells, they can be carried and released to destroy blocks or hit an enemy, they could be flown into the air while being carried aswell.

Super Mario World
Super Mario World brought drastic changes to the Koopa Troopa. In this game, Koopa Troopas finally started walking on two legs and wearing shoes. Also, stomping on Koopas ejects them from their shells, rather than causing them to hide and the shell can be carried. An ejected Koopa will slide out of its shell, wearing only a t-shirt, and will crawl back to its shell after being temporarily stunned. The game also featured Blue and Yellow Koopas. A Koopa that reentered a yellow shell would flash and turn invincible and chase after Mario, sliding around inside the shell. If Yoshi ate a Red, Blue, or Yellow shell, he would gain a different ability until he swallowed or spit the shell out. Eating a flashing shell gave all three at the same time.

Super Mario 64
In Super Mario 64 they wear boxers after being knocked out and then you can use their shells as skate boards.

Paper Mario
The Paper Mario series introduces friendly Koopas that have their own towns, such as Kooper, Koops, etc. However, it also introduced some pretty evil koopas, such as the Koopa Bros. and Tutankoopa.Other forms of Koopas, such as the Koopatrols and the giant Tubba Blubba. Others are Hammer Bros., Lakitu, etc. But, Koopas are nice sometimes. An example of such a Koopa is Koopa the Quick, first appearing in Super Mario 64, who races you in Bob-Omb Battlefield and Tiny-Huge Island. Generic Koopa Troopas have also appeared throughout the Mario Party series as NPCs. Like Toad, they are generally helpful towards the players, usually providing them with rewards or running mini-games.

New Super Mario Bros.
In New Super Mario Bros. they are basically the same as in Super Mario world, they walk on two legs, the wear shoes and can still be carried. Both green and red Koopa Troopas appear and have the same functions as in the other games. The Blue Koopa Shell appears as a power-up

Super Mario Galaxy
In Super Mario Galaxy, they are semi-common enemies found mostly on the airship levels. Strangely enough, they are shown walking on all fours, like in SMB and SMB3, and always show an angry-looking expression. They did not flee from Mario like in Super Mario 64 and instead patrolled a set area; which was a straight line for Green Koopa Troopas and a small circle for Red Koopa Troopas.

Also, unlike in Super Mario 64, Koopa Troopas can actually hurt Mario in this game. As usual, jumping on a Koopa will cause it to retreat into its shell, but they can be defeated permanently by attacking with a ground pound. Attacking with a spin will defeat the Koopa and also spin the shell into Mario's hands so it can be used.

Koopa shells also appear as common items which can be thrown to defeat enemies and smash open chests or held onto underwater in order to swim faster; red shells travel faster than green shells and home in on enemies, whil the green shell go in a straight trajectory.

Mario Spin-offs
Single Koopa Troopas have been playable in Mario Kart games and various other sport games along with Paratroopas. Their shells, both Green and Red, are available as items.

Other appearances
Koopa Troopas were the most regularly-seen minions of Bowser in The Super Mario Bros. Super Show!, where they were voiced by John Stocker, and made one appearance in the Super Mario World animated series. They appear in the popular Super Mario racing video games, Mario Kart.

Trivia

 * In Super Mario Land instead of there shells being kicked they explode.