Man-Serpents are a race created by Robert E. Howard. Appearing in 1952 in his story, The God in the Bowl. The Marvel adaptation were created in 1971 by Roy Thomas and Barry Smith. Making their first appearance in issue #7 of the 1st volume of the Conan the Barbarian comic book series.
The Man-Serpents are descendants of Set, an Old One. It's a race which was spawned by his daughter Ishiti and, prior to the Thurian Age, had splintered from the Serpent Men. The Man-Serpents were a reclusive race, unlike the Serpent Men whom were served by men. When man overthrew the Serpent Men, they though they exterminated the Man-Serpents. In reality, the survivors went into hidings. Its members clashed with Conan of Cimmeria during the Hyborian Age.