One of the staples of the adventure genre is the “Lost Race” story, which grew out of the “Lost World” story and sometimes overlaps quite a bit. Since Sword & Sorcery evolved from adventure fiction, some of that DNA ended up mixed in, but in a kind of interesting way.
Adventure fiction grew out of the 19th century, when a lot of the world map was still pretty blank. Explorers were busy all through the century trying to find all the hidden corners, but for fiction writers there were still plenty of spots to put whatever you wanted, because nobody could say you were wrong. You didn’t even have to give some bullshit explanation as to why your lost civilization didn’t show up on satellite pictures. It was a magical time.
Because all the hidden valleys and lost oases in the world were not as interesting as they would be if there were people in them, pulp writers inevitably populated their forgotten corners of the earth with some advanced civilization, often a remnant of some culture that had existed in the past. These lost worlds were found to contain Romans, Vikings, Cavemen, Israelites, or even Atlanteans. People cut off from the world around them, and somehow still existing on the same cultural and technological plane they had inhabited centuries or millennia before.
This was a very popular subgenre, and a slew of authors cranked out story after story and book after book. Haggard, Kipling, Doyle, Burroughs, and Merritt all wrote books in the genre, and they are just the tip of the iceberg. A lot of these books influenced Howard, and so the idea of the “Lost Race” went into the stew that created Sword & Sorcery, but the nature of fantasy worked a fundamental change.
Because unlike the regular run of Lost Race tales, S&S is not set in the modern world, but in one that is either earlier in history, or in a completely fantastical world with no relation to our own. Thus, the usual candidates for lost races were not available. This means that a writer of Sword & Sorcery who wants to do a lost race story has to invent their own lost race.
In practice this went one of two ways; either the author used some shadowy people out of semi-legendry, or they just invented from whole cloth. Howard himself was quite obsessed with the Picts – who were an actual race who existed in northern Scotland – and used them in several of his Dark Age stories like “Kings of the Night” and “The Dark Man”. To him, the Picts were a pre-Celtic race who had been driven out by later invaders, and who retreated underground to survive, slowly reverting to an almost bestial savagery.
But in a fantasy world, the author has to invent lost races out of nothing. And in Sword & Sorcery fiction this impulse tangles with the Lovecraftian influence to create lost races that are not just alien, but actually inhuman. In a Lovecraftian cosmos, the races that came before man were avowedly subhuman, from the nebulous subterranean beings of Irem, the Deep Ones of Innsmouth, to the Worms of the Earth, the Serpent Men of Valusia, or even the Melniboneans.
Lost Races in Sword & Sorcery fiction are not just remnants of earlier eras, they are enemies of mankind. Inhuman and inimical, serving bestial gods and bent on the destruction of humanity. Inevitably, this makes evil and corruption a racial issue, as dark magic and evil machinations are the heritage of those peoples who descend through aeons of time from some pre-human race of monsters. In a Howard story, if you trace the bloodline of a sorcerer back far enough, you will find a lizard.
This means lost race tales are fundamentally different in S&S. You are never going to have a tale about an explorer who happens upon a lost city of Romans, helps them build a cannon, runs afoul of the high priest, and then escapes hand-in-hand with the princess as the volcano erupts. The only thing remotely like this in the canon is “Red Nails”, and there Howard subverts the tropes by having the hidden race be as bloody-minded and primitive as anyone else in the Hyborian Age. In adventure fiction, a lost race tale makes the modern era look so much more advanced than all those silly primitives. In Sword & Sorcery it rends the veil and reveals civilization as a thin veneer over ages of howling madness.