A "Ghoul" is precisely what Stephen says it is in Telemachus, a "Chewer of corpses." In the alchemy of his imagination, his mother has changed from a dead body to a monster that feeds on bodies. She returns under this guise in Circe, and thoughts of corpses being consumed as food connect with many other reflections on human mortality in the novel.
In ancient Bedouin folklore, the ghul was a wily demonic creature that lived in desolate places, tricking human beings who passed by, killing them, and eating their flesh. It seems that ghouls also sometimes ate bodies from graves, but this did not become their central role until they entered western culture through 18th century translations of One Thousand and One Nights, a.k.a. Arabian Nights. One French translation in particular, that of Antoine Galland (1704-1717), made ghouls into grave robbers. Galland introduced into one of Sheherazade's stories a character named Amina, who tricks a man named Sidi Nouman into marrying her. When he notices that his bride eats nothing but single grains of rice, he follows her out of the house one night and sees her feasting with other ghouls in a cemetery.
Ghouls were shapeshifters in Arabic folklore, assuming the form either of animals (often hyenas) or the human beings they had most recently eaten. Circe shows that Joyce is aware of the tradition of animal-like ghouls eating the dead and assuming their features: "The beagle lifts his snout, showing the grey scorbutic face of Paddy Dignam. He has gnawed all. He exhales a putrid carcasefed breath. He grows to human size and shape. His dachshund coat becomes a brown mortuary habit. His green eye flashes bloodshot. Half of one ear, all the nose and both thumbs are ghouleaten."
Reading retrospectively from this canine corpse-chewer, one may wonder whether Bloom has glimpsed a ghoul in the "obese grey rat" he sees crawling under a tombstone in Hades. Bloom may not know the Arabic mythology that has captured Stephen's imagination, but he certainly thinks of this "old stager" as straddling the border between animal and human: "One of those chaps would make short work of a fellow. Pick the bones clean no matter who it was. Ordinary meat for them. A corpse is meat gone bad. . . . Wonder does the news go about whenever a fresh one is let down. Underground communication. We learned that from them. Wouldn't be surprised. Regular square feed for them."
Stephen has no doubt that his mother has entered a world of corpse-chewing, animal-like specters. When she appears in Circe, "her face worn and noseless," and urges him to repent his loss of faith, he screams, "The ghoul! Hyena!" Whether he thinks that his mother has become a monster, or merely that one has eaten her flesh and assumed her shape, hardly seems to matter. The lines between human and animal, living and dead, nurturing and putrescing, have been erased, in a vivid instance of what Julia Kristeva calls "the abject."
This Arabian thread in the book's manycolored tapestry clearly ties into other reflections on mankind's mortal condition: the continuity between human and animal lives implied by "dogsbody," the cannibalistic possibilities raised by seeing dead flesh as "potted meat," the conception of God as a carnivore, the transformations named by "metempsychosis."
Joyce owned an Italian translation of Le Mille e Una Notte when he lived in Trieste. Where he may have picked up the association of ghouls with hyenas is uncertain.