Shellcreepers are turtle-like enemies from the original Mario Bros. game and are considered to be the predecessors of Koopa Troopas, which they strongly resemble. As with all enemies in Mario Bros., they can only be defeated by flipping them over, either hitting the platform beneath them or using a POW block, then running into them to kick them away while prone. As these game mechanics don't allow enemies to be jumped on (doing so will cost the player a life), Shellcreepers tend to be replaced by Spinies in modern remakes to avoid confusion amongst players familiar with Koopa Troopa behaviour in later games.
Like all the enemies in Mario Bros., if the player doesn't kick the Shellcreeper away after flipping it on its back, it will get up with a different colored shell, and start moving at high speeds. Each time it is flipped over and not defeated, it will move progressively faster.
In Super Smash Bros. Brawl
In the Mario Bros. stage, Shellcreepers are enemies until attacked, at which they retreat into their shells and become throwable items. Shellcreepers, like the other enemies in the stage, cause very large horizontal knockback, whether or not they are thrown. Due to the odd shape and low ceilings of the stage, throwing these enemies is the easiest way of defeating opponents, as there is not enough vertical knockback for them to tech on the ceilings. Sometimes, when they are flipped, their shell will come off, and the player can see them getting up, shell-less, and putting it back on, in a possible reference to Super Mario World. A flipped-over Shellcreeper can be eaten by Munchlax.
As in the original game, each time a Shellcreeper is left on its back for long enough, it will get up again, will change color (green to red, red to blue), and move faster. Throwing these faster Shellcreepers at enemies will cause even more knockback, being a one-hit KO in many situations.
Trophy
Shellcreepers Shelled enemies Mario and Luigi faced back when they were plumbers. They had to be bumped from below to flip them over and then kicked off the stage. The last one on a stage turned from green to red and raced really fast. If one got flipped but not sent off, it would eventually right itself and move at high speeds. They were simply called "turtles" in Japan.
: Mario Bros.
: Super Mario Advance