Dirt eaters are undead creatures seen only in one piece of concept art by Blizzard artist Thammer. They may have been a canceled unit in Warcraft III. Based on the caption, a dirt eater is half buried in dirt when it moves around. They seem to attack by throwing rocks.
Dirteater is one of the many combinations of names granted the permanent pet ghouls under the Death Knight's control via Master of Ghouls.