Hmm, interesting. Research informs me that Hastur was created* first by another author, then Chambers linked "him" into "The King in Yellow", and after that, Lovecraft adopted both.
*Insofar as a creature of such magnitude and unspeakable horror can be created by anything. How unfortunate that I chose to delve into such mysteries just to unravel the secrets that surround the King in Yellow. For now the inner workings of this chaotic world have been shown to me, but too late have I realised that no mortal mind can contain such horror. The mad Arab was right in his writings, if only I had turned back as the professors at the Miskatonic had told me, but now I see all. Azathoth, the nuclear sleeping idiot god that lies a the center of all creation, the veil rended revealing the true form of all... By Enlil and Eniki! Magna mater! Magna mater! Ia Ia Cthulhu fh'tagn...
We regret to inform you that the author of this post has gone mad and is currently undergoing treatment at the Arkham psychiatric ward.