Rubble slimes are cyclopian Slimes that have grown rock-hard shells to protect their gooey innards. The relationship between this species and the Marble slime is unknown.
These rock-hard slimes are renowned for their singe eye and fearsome fangs. It might come as a surprise to learn that they're just as wobbly as their soft-bodied brethren on the inside.
The harsh shading on the monster's original artwork seems to imply the viewer is looking up at it, and that the stem on top of it's head connects it to the roof of a cave like a stalactite.