Okay - one more damn good Fiend before we start fixing up the bad ones.
Grimlocks are, of course, the Fiend Folio's version of H. G. Wells' Morlocks from The Time Machine. They fill that "2 hit die troops" niche that your party starts bumping into once they have a level or two under their belts. Grimlocks are in direct competition with gnolls, troglodytes, lizard men, crabmen and so on.
And they win. Hands down the best 2nd level dungeon soldier.
The Dweller in the Gulf would have been if it had been set on Earth not Mars, and with Earthlings and not Martians populating the strange society the explorers find.
Unlike gnolls and their kindred, unlike the weaker humanoids the party has been battling on the shallow levels, grimlocks are utterly of the underground, natives of its deep places. They may raid on the surface ... but they don't have to. Grimlocks are fundamentally different from the Tolkienoid grunts; imports from the world of science fiction, they nonetheless fit perfectly in a world of weird fantasy.
Completing the greatness of grimlocks is their blindness and Daredevil-sense. Different things work against them. No illusions; no casting light on the face; then again, if they surprise you, it's not because you've been holding lanterns and torches to light your way. Visual dangers of the dungeon - medusa's gaze, bedazzling mosaics, enchanting purple flames - hold no peril for them. But clouds of pepper, or the din of beaten shields? That just might work. One more way their traits and attributes hold together and create a memorable picture of underground horror.
So, one last time - sometimes the Fiend Folio is better.