All three paranormal beings were once humans who died, and passed into living dead status. They are all associated with graveyards, and all things of a non-living nature. Although beyond that each one is quite different from the other.
Zombies are mindless flesh eating drones with a cannibalistic lust for live humans. Their brains in particular seem to send them in a frenzy. Zombies have no intelligent thought, and have a limited shelf life due to their rotting flesh. Although they can extend their life by consuming more live humans. Eventually, in less than a decade, they succumb to the ravages of necrosis, and die once their brain is eradicated.
Humans whose brain by chance survives after being bitten or partially eaten by a zombie will die, and then be reanimated as living dead zombies themselves. Luckily the hard skull makes it difficult to get to the brain. In addition actual dead corpses in morgues, and cemeteries can be made to rise from their graves through the dark magics. Although it’s true that a zombie biting a corpse could reanimate it, they have no taste for their own kind. That being the rotting flesh of the dead.
Also zombies will stagger about in zombie herds as they are instinctively drawn to each other. Their relatively slow speed, intelligence, and easily destroyed flesh make it hard from them to get flesh alone. Even despite their above average strength. In a pack they can overwhelm a person even if they’re armed. Luckily zombies can be killed any way humans can. There’s no need for any special supernatural measures.
The first known zombie was created in approximately 4129 BC when a new embalming process was used on a prominent member of an ancient Sumerian community who was actually still alive. They didn’t have the medical knowledge to know he was actually in a deep coma falling near the edge of death. As the man died his body reacted with the unique set of ingredients that induced Zombification or the birth of the supernatural Zombie virus. He rose from his grave three days after burial and proceeded to cannibalize the community.
Ghouls, unlike zombies, are almost always self-created either purposely or by accident. Either way a ghoul starts out as one sick and/or evil human whose a necro-cannibal, and relishes dead human flesh. However they usually start out enjoying the freshly dead, and once transforming into a ghoul they crave the most disgusting rotted flesh they can get their hands on. The deader the prey, the more power is imbued upon the ghoul as a reward by demonic forces. The act of being a cannibal for several years eventually yields a gradual transformation to living dead ghoul status. Often ghouls start out as human serial killers. If they are captured before becoming a ghoul, they may achieve ghoulness upon death since the beginnings of the demonic DNA may have already formed within them. On rare occasions a ghoul may dine upon an evil corpse, and inadvertently resurrect it.
Now you may think since ghouls only love dead humans this means you’re safe. Well think again. Unlike zombies they are relatively intelligent, and tricky as well. Their mind craves some activity other than eating so hunting down humans, and killing them is something they enjoy as well. They then take the body to a desolate location so it may decay in the sun to their satisfaction.
They can assume the physical form of anyone they eat. For a time they can appear human, and may even be quite charming as they lure their prey into their web of death. Over a short time however their form begins to manifest a monstrous, almost demonic appearance. They then must eat another person to appear normal again. Each feast of death extends their life. If they continue doing this they can live forever.
Since Ghouls can exist in sunlight, although they don’t like it, they are sought out by some vampires for use as servants. Finding the right ghoul can be tough but when it happens the vampires has a loyal servant for life. The ghoul can be an excellent tool for getting blood during the day by luring prey into their dark lairs. Since ghouls prefer to be alone, unlike zombies, a vampire uses his hypnotic charms to keep a ghoul under his control.
Ghouls are very strong, move fast, and have excellent senses along with being hard to kill. Many of the same methods used to kill a vampire will work on a ghoul. The simplest method is to trick it into eating a corpse that is less than 24 hours old. Mixing freshly drawn blood from a person, and mixing it with their undead meal will also yield a quick demise.
Ghouls are thought to have originated with the unholy union of a Jinn, extra-dimensional beings created before humans but after Angels, and Demon who had inadvertently simultaneously possessed an ancient human. So a latent ghoul gene may exist in a great number of people. This would explain how simply eating human flesh would bring on the ghoul condition.
Mummies are usually specifically Egyptian or Chinese in origin, and are humans who were preserved upon death through a combination of chemical preservative, and magical means. They’re almost always royalty, and a curse is put upon their burial tomb where they reside with their treasures ready to take them into the afterlife. However if someone disturbs their eternal rest, then the curse is activated and their corpses reanimate.
They start out as mindless bandaged living dead eating machines out for live human flesh which gradually restores their original appearance, memories, and intelligence. Until they consume enough humans to achieve that, they are just like zombies. Staggering about slowly yet with super strength. Although unlike zombies they aren’t easily killed due to the dark magic that protects them from harm. Eradicating them involves understanding the curse, and combating them through magic.
If they restore their old selves then they must still live the life of a cannibal in order to maintain their youthful appearance, and power as they go through their eternal life journey of mostly evil exploits.
You might also want to check out a previous post that explained the difference between Zombies & Frankensteins, which is another form of supernatural being associated with being resurrected from the dead.