"...the killing had been done by Black Winged Ones which had come to them from their immemorial meeting-place in the haunted wood." The Call of Cthulhu.
And again: "Mankind was not absolutely alone among the conscious things of earth, for shapes came out of the dark to visit the faithful few. But these were not the Great Old Ones. No man had ever seen the Old Ones. The carven idol was great Cthulhu, but none might say whether or not the others were precisely like him."
What are these Black Winged Ones who meet the acolytes of Cthulhu in ye haunted wood? I don't think that they are night-gaunts. Could they be related to the winged things mention'd in "The Festival," which one assumes are black because they are described thus: "They were not altogether crows, nor moles, not buzzards, nor ants, nor vampire bats..." Daniel Harms' The Cthulhu Mythos Encyclopedia does not mention them (I met Daniel in Providence, when he came to buy a copy of Weird Inhabitants of Sesqua Valley). I am curious about all of this because the story I am now writing, for Black Wings III, is a semi-sequel to "The Call of Cthulhu," and I want to mention these murderous Black Winged Ones as they appear to my narrator in dream. I love that Lovecraft mentions these supernatural beasts; for too often some commentators want to stress that such tales as "The Call of Cthulhu" are cosmic but not supernatural--and that is nonsense. The intense stress on dreams, the fiendish Black Winged Ones, and "There were legends of a hidden lake unglimpsed by mortal sight, in which dwelt a huge, formless white polypous thing with luminous eyes; and squatters whispered that bat-winged devils flew up out of caverns in inner earth to worship it at midnight"--all of these make it crystal clear that "The Call of Cthulhu" is first and foremost a horror story.
But what are they, these Black Winged Ones? Did they drift through space & time with Cthulhu when he first secured a foothold on our planet? Were they among the nameless things that built R'lyeh in honor of their great and dreadful lord? I love that there are so many things in Lovecraft, hinted and unexplain'd, that stir the imagination and haunt the soul. Ia! Yog-Sothoth.