Koopa Troopas are common enemies in the "Mario Brothers" series. They are the most common soldiers of the Koopa Troop group. These turtle-like creatures' leader is Bowser, but some Koopa Troopas are tired of this, so some of them do not work for villains, even though they are Mario's enemies. They can easily be defeated by stomping on them then throwing their shell into a pit.
Their ancestors were called "Shellcreepers". These enemies appeared in the game Mario Bros. in 1983. However, they got changed in Super Mario Bros. that was released in 1985.