Oo! A rare opportunity to drop science, before Uke or Auntie Pam get here (has Auntie Pam been around lately?).
HPL is right (as you might expect of a guy named HPL) about Hastur, to my knowledge. He was created by Robert W. Chambers in his book The King in Yellow, is not Cthulhu, but is “he who is not to be named.”
Anyway. While we’re on the subject, I’ll mention that, if we’re beginning from the assumption that the only Cthulhu story worth the name is a Lovecraft Cthulhu story, there really isn’t any such thing as a Cthulhu Mythos. Lovecraft mentioned a bunch of the same entities in different stories, and he and other writers name-checked each others’ creations for what Lovecraft would call (every single chance he got) “versimilitude,” but there really wasn’t all that much attention paid to continuity as we know it in the contemporary fanboy sense. Lovecraft didn’t really build a world like the Star Trek or Marvel Universes. You can blame August Derleth and the rest of the hacks-what-came-after for the idea of a Cthulhu Mythos. (Admission: I’ve never read any of Derleth’s stuff, or indeed any non-Lovecraft “Mythos” stories at all, but the consensus seems to be that Derleth, though he gets a big yay for keeping H.P.L.'s work in print, pretty much missed the point, creatively, when writing “in the tradition” of Lovecraft.)
Yours in run-on sentences,