The walking un-dead have been a staple of horror since their introduction into film in 1932 with White Zombie, starring Bela Lugosi. Though the zombies here were quite harmless, replacing slaves, the idea stuck, and before long they've evolved into an entirely new set of enemies for horrified protagonists to slaughter, ranging from the slow and lumbering to the quick and agile.
Joining the ranks of vampires, werewolves, the Frankenstein monster, mummies, and bestial alter egos, the zombie's origins give it a quality that makes it stand alone. They "possess an alleged basis in fact or, at least, Afro-Caribbean superstition. Zombies are held to be corpses that have been re-animated by voodoo rituals in order to serve as a source of cheap labor for the owners of cane plantations and sugar mills in Haiti" (Frank 189). Other common monsters are merely the product of an imagination, and given each are given their own fictional story and creation. Zombies are something adapted from real life to the silver screen.
White Zombie turned out to be extremely well timed. It was at this period that horror films began to explode in number. "Horror, edited by Phil Hardy, lists a mere 28 horror films made before 1920; 50 made in the 1920s, 65 in the 1930s; 93 in the 1940s; 106 in the 1950s; 306 in the 1960s; 470 in the 1970s, and 210 from 1980 to 1984" (Klossner 426). In sixty years, that's a difference of a little less than one horror movie every two and a half months to one about every seven and a half days.
While the same rise-and-murder principles govern two other common monsters, the mummy and the golem, zombies are different. The golem is a creation, something assembled from inanimate objects, something no one that interacts with it was ever related to in any way. An extra element the zombie has is that (almost required in films) one of them could be someone you have once known, and you will be forced to attack and kill what you recognize as a friend or family member. While mummies were once living beings, no one has ever known them or shared an emotional connection; they're much easier to vilify. The requirement for being turned into a zombie is also different than that of a mummy: "it isn't just dead nobles who can be roused from their eternal slumbers -- even the average Joe or Jane may become a reanimated corpse if circumstances are favorable for the creation of zombies" (Fonseca 59). While zombies can also be raised like mummies by a mystical curse (Evil Dead for example), these "favorable circumstances" can come in the form of a virus, like in Resident Evil and 28 Days Later, voodoo in White Zombie, and radiation from a space probe in Night Of The Living Dead.
Of course, once free of their crypts and coffins, zombies themselves also come in a wide variety. A variety that's gaining popularity is ones that have all the physical attributes of their human hosts, usually ranging in strength and speed. These are pushed to the extremes in such films as 28 Days Later and the 2004 remake of Dawn Of The Dead, although they can be seen acting more like the everyday people they embody and chase in the Evil Dead series. Then there's the slow, lumbering, brain-dead variety, which have proven the most popular. These can be seen in Resident Evil, and the recent comedy Shaun Of The Dead.
Originally, the psychological appeal of zombies came from the dead returning to life, and resenting the disturbance to their eternal slumber: "the dead are raised by the living who simply cannot accept the finality of death, as in Stephen King's Pet Semetary" (Fonseca 60). Now, however, they've grown to be a shortcut to violence: "its successes are as dangerous and horrifying as any movie monster, and often as gory in their activities" (Frank 189). A viewing of the 2004 remake of Dawn Of The Dead would make one argue that a zombie movie can surpass a normal movie in blood, gore, and violence, as the main characters use chainsaws, dynamite, rifles, shotguns, and various other assorted tools to dispatch their would-be predators. "The modern period, which began in 1957 ... has been characterized by pessimism, cynicism and, above all, explicit violence" (Klossner 426). Whenever a viewer sits down to a zombie movie, especially recent releases, they can expect to see unparalleled amounts of blood, realistic flesh rotting and consumption, and deaths that gain popularity by being increasingly gruesome.
Rather than work on the psychological effects of vampires that drink blood, golems that have souls from an unexplained source, or half-men half-wolves, zombies have made their claim to part of the horror genre by attacking, and being slaughtered, en masse.