Koopa Troopas are an enemy from the Super Mario Bros series, and are the common foot soldiers of Bowser. With their sharp beaks, Koopas resemble snapping turtles. The most common Koopas have green or red shells, though blue and yellow variants did appear in Super Mario World. The color of a Koopa's shell determines its behavior: green-shelled Koopas will always walk in a straight line, while red-shelled Koopas will turn around to avoid a pitfall. Other variants include the winged Koopa Paratroopas that are able to fly, the skeletal Dry Bones, and the cape-wearing Super Koopas of Super Mario World. The Hammer Bros, Magikoopas, and Lakitu are related to Koopa Troopas, albeit with more dangerous abilities. Possibly at the top of the Koopa hierarchy are the Koopa Kids, Bowser's children who first appeared in Super Mario Bros 3 as the bosses of the game's eight worlds.
When they first appeared in Super Mario Bros, Koopas were vulnerable to a stomp attack from above. They would retreat into their shells, which could then be kicked along the ground. Super Mario World brought a stylistic change to the Koopas when they were first shown walking on their hind legs and in shoes. When stomped they would be ejected from their shells, dressed in white boxer shorts and t-shirts. Since Super Mario World, Koopas have continued to walk upright. Outside of the main Super Mario games, Koopas have made appearances in off-series games like Mario Kart, Paper Mario, Mario Party, and the Mario sports titles.
Mario Bros featured an enemy called the Shellcreeper, which looked identical to a Koopa Troopa. Though they could not be stomped from above, this is arguably the first appearance of the enemy that would come to be known as the Koopa Troopa. Even after being struck from below, a green Shellcreeper would walk out of its shell (wearing white boxers and t-shirts,) kick it over, turn red, and pursue Mario and Luigi faster than before.