Crab is a scrapped Badnik that was originally going to appear in Sonic 3D: Flickies' Island.
For many years, the only evidence of this Badnik appearing in the game was through an early screenshot that is thought to be a mockup, as it is wildly different to anything seen in the final product. Its appearance is similar to that of a Shellcracker.
Over 20 years after Sonic 3D's release, this long-lost Badnik was finally found in a specially-written level editor build of the game by Jon Burton, the co-founder of Traveller's Tales and the game's original programmer, after someone had sent him the early screenshot. The Badnik was restored via the Sonic 3D: Director's Cut ROM hack, where it can be found scuttling back and forth in Rusty Ruin Zone and releases a red Flicky when destroyed.