I normally attribute Hastur, The King in Yellow, and the Yellow Sign with Derleth, and I’ve said before what I think of him, but really, I should look more kindly on the yellow garbed one.
Hastur was first written about in Ambrose Bierce’s short story, “Haita the Shepherd”. In this story, he was the god of shepherds, and actually quite a nice guy. Several of the names of people and places in the collection that this story first appeared in would go on to be used along side Hastur in future stories by other writers. The first of which was Robert W. Chambers, who wrote a book called, The King in Yellow. This was a series of short stories centered around a play of the same name, a play, which causes madness in anybody who reads it. This series of stories is also where we first find the yellow sign. Much like the Elder Sign, there is some contention as to what it actually looks like, especially since Chambers never said what it actually looked like. You can read a little more, on that here.
Lovecraft was a fan of The King in Yellow, and would use Hastur and the Yellow Sign, as well as other related names and places, in a few books. Never really adding all that much in detail, more or less just using the names in homage to another writer. Lovecraft and his circle of friends were all pretty big on this when they wrote, borrowing names and themes from each other. It’s the sort of thing that writers couldn’t even think of doing today without being sued into the ground, but it really worked for them.
Essentially, Hastur started off being Bierce’s benevolent Shepard god. Then Chambers decided that he liked some of the names, and incorporated them into his stories about madness. Next Lovecraft decided to tip his literary hat to Chamber’s earlier work and include some mentions to the earlier works. And lastly, Derleth came along, and decided that there were a lot of blanks that needed filling in, and like an eight year old with a stack of madlibs and a pound of meth, he filled those blanks in.
Derleth made Hastur the half brother of Cthulhu, and somehow assigned him as an air elemental (Cthulhu was a water elemental, in his scheme, air and water were opposites. The only way I would ever accept this version of Yog Sothothery, is if they released a Lovecraftian edition of Battle Beasts.)
Later, he would get bastardised even more as Chaosium would release their Call of the Cthulhu RPG. They would merge the byakhee of Derleth’s creation with the creatures from Lovecraft’s “The Festival” into one race of creatures subservient to Hastur, and even write previously unwritten play from Chamber’s early collection (their version did not cause the madness that came with Chamber’s vision though).