Kilmore said:
The Derro were in literature too. Some 30's science fiction book I believe.
Actually, it was a whole series of stories and articles by Richard Shaver published in various magazines edited or published by Ray Palmer, starting in the late 40's. I don't know whether
Dexter's Lab may have gotten the name Mandark from these as well.
The stories were fictionalized accounts of what Shaver actually believed to be true about the earth, which he felt had an extensive underdark many times larger than the surface. Doug Skinner (the frightened guy in the men's room stall in
Crocodile Dundee II) described the stories this way:
"He claimed our world was honeycombed with huge underground caverns built long ago by Titans from another galaxy. When these godlings fled to escape the radiation of our aging sun, their castoffs degenerated into evil dwarves Shaver called "dero." (For the record, he pronounced this "day-ro."' In "Mantong" -- according to Shaver, our original language -- "de" means "detrimental energy," "ro" means "slave." A dero is someone who is compulsively evil.) These nasty gnomes prey on surface humans, toy with the elaborate technology abandoned by the Titans, and trade ancient machinery and abductees with touring extraterrestrials. Shaver insisted he'd visited these primeval caverns, and poured forth his exploits and assertions in a stream of stories and articles."
The full article can be found
here (warning: Shaver's very stylized illustrations contain some nudity. But that's just a sword hilt in the second one.)