Rock People

The Rock People are characters who appear in "The Chamber of Frozen Blades". Ice King takes Gunter to a hospital in what appears to be the Rock People's city. The impatient Ice King freezes several Rock People who are waiting in line.

They were also supposed to appear in "It Came from the Nightosphere", according to the Frederator blog. However, they were apparently cut out of the actual episode.

Appearance
They resemble rocks with arms, legs and faces.