"They are neither man nor woman
They are neither brute nor human
They are Ghouls"
— Edgar Allan Poe
What are ghouls? A ghoul is a legendary evil being that robs graves and feeds on corpses. It is one who shows morbid interest in things considered shocking or repulsive and supposedly lives in burial grounds. In Arabic folklore, ghouls are a type of jinn that could change their shapes but had one unchanging feature: donkey's hooves for feet. Even more horrible, it kills young children and even can lure unwary folk into abandoned places. By extension, the word ghoul is also used derogatorily to refer to a person who delights in the macabre, or whose profession is linked directly to death, such as a gravedigger.
Ghoul is from the Arabic ghul, from ghala "to seize." It is even thought to come from Gallu, a Mesopotamian demon. In Sumerian and Akkadian mythology, the Gallus (also called gallu demons or gallas [Akkadian: gallû]) were great demons/devils of the underworld. Their job was to haul off unfortunate victims to the underworld and even accompanied Istar when she headed down to the underworld.
The ghul is a fiendish type of jinn believed to be sired by Iblis, known to the Devil in Islam. A ghoul is a desert-dwelling shapeshifting demon that can assume the guise of an animal especially a hyena. It lures unwary people into the desert wastes or abandoned places to slay and devour them. The creature also preys on young children, drinks blood, steals coins, and eats the dead, taking the form of the person most recently eaten. In the Arabic language, the female form is given as ghouleh and the plural is ghilan. In colloquial Arabic, the term is sometimes used to describe a greedy or gluttonous individual.
In stories and films, I think Gollum as closest to the idea of a ghoul, and wouldn't be surprised this is what Tolkien had in mind when he wrote him into The Hobbit and The Lord of the Rings trilogy.