Morlock

Morlocks are subterranean predators which HG Wells predicted in his satirical novel The Time Machine, whose prey is us here on the surface. Wells had delved into all manner of European lore about trolls under bridges, dwarves in the mountains, and the svart alfr.

Morlocks joined gremlins as neomythology when the derro came into our knowledge. Dungeons & Dragons has several morlock-like monsters, not least those derro, but also the Fiend Folio grimlocks and maybe meenlocks which precede even them. But it didn't get around to including morlocks themselves.

Perhaps TSR worried about copyright, perhaps TSR figured these beasties - as a future race - unfit for a mediaeval setting. (As if that stopped other time traveling races like illithids, or all that scifi crap in City Of The Gods and Barrier Peaks.) Paizo got a clue or, maybe, figured out that by the late 2000s all that Victoriana was public-domain; so included them in Pathfinder.