"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, nor 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; and as they reached the throng of celebrants the cowled figures seized and mounted them, and rode off one by one along the reaches of that unlighted river, into pits and galleries of panic where poison springs feed frightful and undiscoverable cataracts...."

Another Lovecraft creature! This time it's from his short story "The Festival". It's a pretty short story, but in it describes these grotesque looking mounts. So here's my vision of them.