In Litany To Hastur XX! The Unspeakable, Lin Carter describes the Byakhee as bat-winged, black-furred, iron-beaked and with "eyes of hell". The Encyclopedia Cthulhiana describes them as "creatures resembling bats, birds, moles and rotting corpses, but not quite like any of them." August Derleth describes a Shantak in much the same way, so the two may be the same species.
In Litany To Hastur XXI The Unspeakable, Lin Carter has the Byakhee dwelling upon the "cloudy shores of Lake Hali"; whilst in Carcosa Story About Hali, he mentions nests of the Byakhee on the planet Carcosa. In The Festival, they are present at a cult celebration in a cave beneath Kingsport. Other sources, such as The Things That Flap Above imply that they dwell in the city of Carcosa. Some sources indicate that they can be found dwelling on various planets in outer space. It is probable that "the winged races of the Hyades" in The Colour Of Death are the Byakhee.
"Byakhee were once intelligent inhabitants of a city drawn into Carcosa, an experience that drove the species mad. Some say they made a deal with Hastur to support them in a conflict that led to their homeworld's destruction. The byakhee now dwell in Carcosa and interstellar space, though they have been known to frequent Yuggoth and other worlds.
"Byakhee serve the Great Old One Hastur... They are also served as mounts that may carry a rider between the stars...
"Call of Cthulhu has taken its description of the Byakhee from the creatures in Lovecraft's The Festival, but it uncertain whether Lovecraft and Derleth's creations are one and the same."
In this story by H.P. Lovecraft, riding beasts are described that some have identified with the Byakhee. They are, however, not described in a way that implied intelligence, unlike many depictions of the Byakhee. Nor are they particularly humanoid.