Huh, never realized that the Grimlocks are pretty much the Morlocks from H. G. Well's "The Time Machine". I kinda disagree with the above, that there are only really two categories for evil. Even then though, they could fit into the first category a more subtle way. You're would have to really play up on the Blindsight though. Besides, if their head is a Cleric to the god of slaughter, you've got your supernatural right there.
You've probably never seen them used as main villains because they lean more towards henchmen.