Shellcreepers, along with Sidesteppers, first appeared in the arcade game Mario Bros. as enemies of Mario and Luigi. They would emerge from the pipes on the upper portion of the stages and move from one side to the other. Much like their appearance in Brawl, when they are not kicked after being hit from below, they change colors and become faster.
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. When they are left flipped for a bit, they'll take their shell off, put it upright, and put it back on, just like they did in the original Mario Bros. game. 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.
Since the Mario Bros. stage was removed in Smash 4, the Shellcreepers have only a trophy in the Wii U version of Smash 4.