From the Super Mario Wiki
An Attacky Sack is a large, rubbery, Chomp-like enemy from Yoshi's Story. It has small eyes and long sharp teeth; its body is covered in multiple patchy colors. It bounces off of walls and can be a real nuisance in tight spaces, especially because of the fact that it is invincible. Magma Castle, the final level of Yoshi's Story, is the only place in the Mario series in which they can be found.