Kirby RPG and the Infinity Shard
A major game that shellcreeper's appear in is Kirby RPG and the Infinity Shard, where they are led by Waddle Doodle.
They are a enemy in the classic game, Mario Bros. They come out of the pipes. The way to kill them is to hit the spot where they are under. They look like Koopas. They can also die to the POW Block.
One main Shellcreeper character is Joe Koopa, there are also many other shellcreeper characters in Kirby RPG and the Infinity Shard, such as The Shellcreeperville Elder, and villagers.