Byakhee are an extraterrestrial race known from the Cthulhu-Mythos. They are servants of Hastur the Unspeakable.
Lovecraft himself only mentions them in his story The Festival, where they are described as follows:
- "A horde of tame, trained hybrid winged things that no sound eye could ever wholly grasp, or sound brain ever wholly remember. They were not altogether crows, nor moles, nor buzzards, not ants, nor vampire bats, nor decomposed human beings; but something I cannot and must not recall. They flopped limply along, half with their webbed feet and half with their membranous wings…"
- ―H.P. Lovecraft - The Festival
While Lovecraft doesn't name the flying creatures in his tale, they are mostly associated with the Byakhee, which are bat-like birds described by August Derleth in The Trail of Cthulhu.
The creatures in The Festival were capable of interdimensional travel. Derleth's Byakhees behave somewhat different, here the rider has to drink space mead before he calls them in his dreams through whistles and chants. Then the creature carries him to another place or time. The space mead most likely protects the rider from the negative impacts interdimensional travel would have on the human body.
To reach their destinations, the Byakhee have an organ called hune, that is “attuned to the galactic magnetic field.” It creates a space-time-anomaly called keim, in which the Byakhee could move with up to 400 times the speed of light.
Curiously, a galactic magnetic field was only discovered as recent as 2015, while The Trail of Cthulhu was published from 1944 to 1952.