I'm gonna go with the traditional ghoul, since it seems more plausible.
I've done a lot of speculation regarding humanoid fantasy creatures, coming up with explanations for the evolution of beasts such as the troll, elves, dwarves etc.
So, in all of these postulations, the theory has always been that the creatures diverged from humans. So, since ghouls are also very humanoid, I presume that their evolution would branch from that of the hominids.
Most depictions of ghouls show them as hairless, pale-skinned humanoids with sharp teeth. So what we need to figure out is:
1) Why did they lose their hair?
2) Why are they pale-skinned?
3) Why are their teeth sharp?
Now we must look to the real natural world to help us figure this out. Let's start with number one, because... well, because it's number one.
Humans naturally have hair, though it can be removed, so the ghoul would have had to had a reason to evolve hairlessness. To answer this question, we can look for a bald creature in nature that has evolved from furred ancestors. The first creature that comes into my mind is the naked mole-rat, Heterocephalus glaber.
This is a naked mole-rat:
And this is a normal mole-rat, or blesmol:
So, as you can see, the naked mole-rat lost its hair for a reason. It lives underground in a hot climate - this eliminates the necessity to protect from cold or the sun. So, we can speculate that your ghouls have a subterranean habitat too.
Now we've got the problem of pale skin. When I think of a pale-skinned cave-dwelling animal, the olm comes to mind.
It lacks a lot of the pigment riboflavin, and is thus pale in colour, even translucent in places. I haven't been able to cross-check it with any written online source, but I presume this is because of the darkness in caves. There is little light, and therefore no need for pigmentation. However, the olm has also gotten to the stage of evolution where it is blind. I cannot find any source to state that pale skin evolves sooner than blindness, or if one is dependent on the other, so I'm gonna have to use the information I know for sure and say that, possibly, white skin evolves alongside, or after blindnes.
Basically, that means that your ghouls could be blind, perhaps even eyeless. But, I presume you are using these ghouls for a horror story, or scary element to a story, so that might be a great idea to make them even more terrifying. What's more, most depictions of traditional ghouls online show them with white eyes, which could be an intermediary stage between sight and eyeleness. This might also make sense, since humans evolved fairly recently, so ghouls would've evolved even more recently.
You may be wondering why I'm going to so much trouble explaining why these ghouls have pale skin - don't (White) humans have pale skin anyway? Well, earlier I said that they would have to live in a climate around as hot as the naked mole rat's range (East Africa), which is a lot hotter, usually, than you could get in Europe, home of the Caucasians.
Now, hairlessness; check, pale skin; check (plus blindness), now all we need is sharp teeth.
It is a relatively well-known fact that herbivores have blunt teeth and carnivores have sharp ones. Here is a domestic cow tooth, compared to that of a lion:
Ridged, blocky teeth are good for grinding plant fibre, while sharp teeth cut through flesh like no other shape. So, your ghouls are gonna have to be carnivores. So, unless a whole ecosystem of megafauna has evolved without sight in a cave system, which would be cool, but perhaps to "monumental" just to justify a creature in the plot, the ghouls will have to come out of the cave to eat meat.
If you want to justify the hairlessness, they'll have to do it at night, somewhere either with a warm night temperature (Not sure if that exists), or just for brief periods of time.
One carnivorous cave-dwelling creature that comes out of its cave at night is the bat. They are not blind, as rumours would have you believe, but have the same level of night-vision as us - which is pretty terrible as animal sight goes. Obviously, they navigate with echolocation, by sending out sound waves of sound that bounce back to them. But for that, you need massive, massive ears - which ghouls lack.
So I advise that your ghouls use a very good sense of smell instead. But, we still haven't figured out why their fully carnivorous. Unless there's some strange event in your ghoul's habitat that kills massive numbers of animals daily (Which only the most biodiverse of ecosystems would survive, considering the accumulative scale of the necropsy.), these ghouls can't survive on just carrion. They'll need to be hunters.
Humans, as a species, are pretty terrible at hunting without tools, so ghouls would be faster and stronger than us. This whole predatory thing could add another chilling element to your story - the thought of a pale, eyeless human killing a deer is rather sickening.
At last, we're done! Sorry for the gigantic answer, but I hope it's been fruitful. To summarise your ghouls, they are a species diverged from Negroid Homo sapiens sapiens, driven into subterranean life by predators, discrimination or other threats, and they have the following features:
Well that's enough from me. Happy worldbuilding!