From the Super Mario Wiki
An Attacky Sack is a large, rubbery, Chomp-like enemy from Yoshi's Story. The name is a pun on a hacky sack. It has small eyes and long sharp teeth; its body is covered in multiple patchy colors. It bounces off of walls and is invincible. Magma Castle, the final level of Yoshi's Story, is the only place they can be found.