Crawlton, called Zakin (ザキーン) in the Japanese manual, is a badnik from the 16-bit version of Sonic the Hedgehog 2 found in Mystic Cave Zone. It is a long blue centipede-like creature that springs out of platforms and ceilings to attack you when you get close. It is possible to see them inside their platform if you keep an eye out. The only way to destroy them is by hitting the head.
It shares some similarity with the Caterkiller from Sonic 1, though isn't quite as dangerous as its 'cousin'.