Ghouls are a species of cannibalistic nocturnal creatures from the Cthulhu Mythos. They are first described in the Cthulhu Mythos by Lovecraft's story "Pickman's Model".
Lovecraftian Ghouls are usually described as white or green-skinned humanoid hairless creatures with long canine muzzles, pointed ears, and clawed feet that have almost become hooves. They inhabit networks of underground tunnels and crypts, and eat the corpses of dead humans. Despite their favored food and reclusive habits, Ghouls are usually not hostile creatures, and in "The Dream-Quest of Unknown Kadath" the protagonist gains the aid of a group of Ghouls and travels with them for a period of time. Ghouls communicate using a "meeping" or gibbering vocalization called Ghoul Speech or Pnathic.
Ghouls are found through the underworld of the Dreamlands as well, and it seems that they can navigate between the waking world and the dreamlands through the use of special tunnels. The universal center of ghoul activity seems to be the bone-filled Vale of Pnath in the Dreamland underworld, were the world's ghouls dump cleaned bones. This dumping is done from the "Crag of the Ghouls", a cliff jutting off from the mountains of Thok over the vale.
Ghouls are known to worship the Great Old One Mordiggian, and his priests wear concealing hooded purple robes and metal skull-shaped masks, which double as disguises when the ghouls wish to walk among humans.
Ghouls appear to be a separate species from humans, breeding and living as their own society, but it also seems that some humans can slowly become ghouls, though the exact means are unknown.