Shellcreepers, along with Sidesteppers and Freezies, made their debut in Mario Bros.. They are turtle-like creatures and are considered to be the predecessors of Koopa Troopas, which they strongly resemble. Unlike the Koopa Troopas, however, the Shellcreepers cannot be defeated by jumping on them and will cause the player to lose a life on contact. Also, Shellcreepers can attack by biting the player.
Shellcreepers emerge from the pipes on the upper portion of the stages and move from one side to the other. They can be defeated either by being hit from below or is turned upside-down by the POW Block, after which they can be kicked off-screen. If the Shellcreepers are not defeated while flipped upside down, they take their shell off, put it upright, and put it back on, after which they change color and moves faster.
As a stage hazard
Shellcreepers appear as a stage element in the Mario Bros. stage. They are stage hazards until they are attacked or hit by the stage's POW Block, at which point they retreat into their shells and become throwable items. As items they can also be hit to send them flying: weak attacks like jabs or Fox's Blaster launching them a short distance and strong attacks will launch them very far.
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. A flipped-over Shellcreeper can be eaten by Munchlax.
Similar to their behavior in the original game, each time a Shellcreeper is left on its back for long enough, it will get up again, will change color (from green to purple to red), and move faster. Different colored Shellcreepers don't have any differences when hit or when used as an item.
The Shellcreepers appears as part of the Mario Bros. Trophy Box.
Shellcreepers return as a stage hazard in the returning Mario Bros. stage. Their behavior when walking around is unchanged, however how knockback affects them has been changed. In Brawl any attack was able to flip a Shellcreeper and launch them, now they receive knockback similar to characters and need some amount of knockback to flip. This prevents things like Fox's Blaster or Bayonetta's Bullet Arts from protecting themselves from Shellcreepers or having a low-commitment method of launching them at opponents. Low knockback attacks like Falco's Blaster can still flip them, but they will need to be picked up and thrown or hit with a stronger attack to be of any offensive use.