All subjects belonging to the Cthulhu Mythos
(created within the Mythos or introduced into it).
The "Cthulhu Mythos proper" coined by Robert M. Price was shaped by H.P. Lovecraft, developed by himself and other authors; the Lovecraft Circle, comprised of Lovecraft's friends and correspondents Clark Ashton Smith, Robert E. Howard, Robert Bloch, Frank Belknap Long, August Derleth, and himself (but can be expanded to other friends and correspondents, such as E. Hoffmann Price, or Henry Kuttner). According to Robert M. Price, that Mythos could be divided into three separate groups: the Dunsanian, Arkham, and Cthulhu Mythos
The Cthulhu Mythos was created by Derleth, was the merging of the writings of the Lovecraft Circle.
For the same reasons, the subjects belonging to Conan, Kull, Solomon Kane, Bran Mak Morn (creations of Robert E. Howard), Thongor (creation of Lin Carter), etc. stories won't be included unless they are closely tied to any of those Mythoses.)
That Mythos also included elements created by ealier authors, such as Algernon Blackwood, Arthur Machen, Robert W. Chambers, or Ambrose Bierce, and was continued with many others since, such as Lin Carter, or Brian Lumley.
For the sake of relevancy and exhaustivity, this categories also includes the subjects involved in Lovecraft's "Dunsanian"/Dreamlands cycle, the Tsathogguan or Hyperborean Cycle of Clark Ashton Smith, and the Legend of the Elder Gods; and in other Mythoses, or Mythoi (or their subsets of Mythoses), such as the Carcosa Mythos, or Yellow Mythos, referring to the creations of Ambrose Bierce, later used by Robert W. Chambers, and close from the Hastur Mythos coined by John Tynes.