Ghoul

Ghouls are monsters that have appeared in a wide variety of forms throughout roleplaying history; if you have a fantasy or a horror setting, then you're very likely to encounter a ghoul in some form. Taking their name from "Ghul", an Arabic evil spirit or djinn that haunted graveyards and feasted on the corpses interred there, ghouls are typically undead creatures that are driven by an insatiable urge for flesh. Ardent necrovores, most forms of ghoul in /tg/ media also happily kill living beings to produce more corpses for them to feed on.

The "iconic" ghoul in most minds, based on the Dungeons & Dragons version, which is in turn based on the H. P. Lovecraft version, is actually fairly close to the iconic "Romero Zombie"; an undead flesh-eater that ceaselessly hunts for prey and which carries debilitating diseases that can cause those it wounds or kills to eventually become ghouls themselves. This is actually quite fitting, as Romero himself referred to his original movie's monsters as "Ghouls", as at the time people only knew zombies from their role in Haitian mythology as undead automatons.

All Flesh Must Be Eaten

In addition to the ability to reference Romero and call any zombies you create "ghouls", actual ghouls appear in the Atlas of the Walking Dead sourcebook. They are portrayed as intelligent, rational, unearthly beings with the ability to shapeshift between human-like and zombie-like forms, magical skills, and a need to feed on corpses.

Call of Cthulhu

As per their depiction in H.P. Lovecraft's books, Ghouls in the Cthulhuverse are a fairly benign race of alien creatures who look like someone mixed a humanoid with a dog and a carnivorous donkey and then allowed it to get slightly corpse-like. They feed exclusively on dead flesh and can transform willing humans into members of their own race, but have little malign intent towards humanity.

Probably. I mean, there're stories...

Dungeons & Dragons

The most iconic form of ghoul in many eyes, D&D ghouls are undead who can spread through an infectious disease they carry in their fangs/claws, be created by necromancers, or spontaneously arise from the bodies of cannibals or evil gluttons. They are faster, more agile and more intelligent than zombies, with at least a bestial level of intelligence, and a genuinely macabre and malevolent nature. They are perhaps most infamous for their paralyzing touch, which lets them freeze your PC immobile so they can then eat them alive; this notoriously cheap ability has frustrated many players, especially because, traditionally, elves are immune to its effects for no discernible non-fluff reason. Ghouls also have more powerful versions, such as the Ghast (which is basically a leveled-up ghoul with a permanent stinking cloud aura who can paralyze elves) and the Ghoul Lord, and aquatic versions called Lacedons.

Warhammer Fantasy

In the Warhammer Fantasy world, ghouls are the corrupted descendants of humans reduced to cannibalism, something that irrevocably tainted them to the point they are not undead, but close enough to it that they instinctively obey the Vampire Counts.

World of Darkness

Ghouls in the World of Darkness have no real resemblance to any of their cousins mentioned here, and are mostly named as such because "Renfields" was a stupid name for a minor template. In essence, in Vampire: The Masquerade and Vampire: The Requiem, vampires who feed a mortal regularly with their blood can give that mortal ever-lasting youth, a vampire-like healing factor, and technically minor access to vampire powers. Why would they do this? Because it also puts a mind-whammy on the ghoul compelling them to love, honor & obey the vampire with all their hearts, and with the added stick that missing a regular feeding will strip the ghoul of all its powers and cause age to rapidly catch up with them, vampires find ghouls essential as the closest things they can know to loyal, trustworthy servants capable of operating during the day.