I've been re-reading some childhood favorites lately and found this:
"His sight came back but now he was held by clammy hands and when he saw his captors he shuddered. Shadowy creatures of Limbo held him— ghouls summoned by sorcery. Their dead faces smiled but their dead eyes remained dead. Elric felt the heat and the strength leaving his body and it was almost as if the ghouls sucked it from him. He could almost feel his vitality traveling from his own body to theirs." — Michael Moorcock, The Vanishing Tower (Elric part IV)
And then on the next page, this:
"Elric was gasping as the last of the heat fled his bones. He now could not stand, but hung in the arms of the dead creatures. Theleb K'aarna must have planned this for weeks, for it took many spells and pacts with the guardians of Limbo to bring such ghouls to Earth."
So the major ability of the ghoul comes from Moorcock, but the ecology of the ghoul originates elsewhere. Ghouls are all over the place in D&D, and are presented as (more or less) naturally occurring undead savages rather than summoned servants of powerful wizards.
According to the various editions' monster manuals, humans can become ghouls either by practicing cannibalism in life or by being killed by a ghoul. The second part is easy enough to source; Night of the Living Dead came out a few years before OD&D, and the word zombie doesn't appear anywhere in that film. Instead, Romero's monsters were originally referred to as ghouls. Gygarneson took what was useful (disease spread by bite) and discarded everything else (slow moving, mindless things ideal for target practice).
The idea of transformation into a ghoul probably comes from Lovecraft. He never explicitly stated that cannibal humans become ghouls, but he never explicitly stated much, preferring to sort of vaguely indicate impressions and let the reader fill in the gaps. A few of his stories that involve ghouls leave an impression that humans can become ghouls through some abhorrent behavior or other, and it wouldn't have been much of a stretch for Arnegax to decide on cannibalism, since that was the abhorrent behavior that Lovecraft's ghouls engaged in after the transformation.
And that is where D&D ghouls come from.
