A similar theme is explored in ‘The Shadow Kingdom’ featuring Conan’s literary and historical predecessor, King Kull of Volusia. Kull, and Atlantean barbarian ruling the ancient and decadent mainland Thurian kingdom of Volusia (much as Conan would rule the Hyborian realm of Aquileia thousands of years later), finds himself caught in a horrifying plot as members of his court are revealed not to be who they seem. They are instead the pre-human Serpent People of Volusia, who can could warp the minds of Men to appear human themselves. Formerly the dominant civilization of Earth when the ancestors of Men were mere ape-like savages, for ages they have secretly ruled the world of Men in this occult and sinister way. Now Kull and his faithful Pictish ally Brule the Spear-Slayer must fight this ancient evil if the destiny of Men is to be restored to them…
Howard never explicitly states that his Valusian serpent people are those of the Worms of the Earth, albeit the latter in a degenerate and declining form. But considering that all of his tales seem to be set in the same universe, that seems a plausible interpretation.
Of course, Howard did not invent the concept of pre-human serpent people, for which there are many mythological analogues including the Nagas of Indian myth. But he did perhaps present one of their most interesting presentations in modern-day fantasy literature.
Oddly, the scenario described in The Shadow Kingdom seems to have been replicated without credit by certain ancient alien conspiracy theorists today. I will leave this for others to ponder further, but one suspects Howard would have wanted some credit for his ideas.