This post begins: L. Sprague de Camp found himself in the position of editor and collaborator on the Conan series after 1951, when he read Conan the Conqueror. He had to revise all the old stories as well as produce new ones. These new stories that had never appeared in Weird Tales in 1932 to 1936, were rewrites of non-Conan tales, many of them historical pieces originally intended for Adventure. De Camp did this by changing names of locales like Egypt becoming the Hyborian Stygia. He also had to add a supernatural element, usually featuring a monster of some kind. (Just to be fair, occasionally he did not add a monster as in “Hawks over Shem” (Fantastic Universe, October 1955). These new monsters were often not that new. Apish or demons of the air, De Camp did a good job of using monsters that Howard himself might have created.
If you’d like to read the rest, please check out Monster: From the Pages of Dark Worlds Quarterly.