Mundanger

"Rich: I'd ask you this question, Stew: who's really the worst monster in this so-called world of ours? Is it really the Medusa, the snake-haired temptress of olden times? Or is it another monster, Stew? A monster maybe a bit closer to home? A monster we call... man? Stew: It's the medusa with all the snakes in her hair. That's the monster. Rich: Yeah. Stew: Yeah, it is, yeah. Rich: Do you want to watch that celebrity arse video now?"

- Richard Herring and Stewart Lee, Lee And Herring's Reasonably Scary Monsters

If your world is a Fantasy Kitchen Sink where the heroes spend every week battling vampires, aliens, ghosts and fairies, one easy way to mix things up a little is to scrap the supernatural element altogether for a chapter and have the heroes fight something relatively mundane, be it a serial killer, a robber or even just a murderously grumpy animal.

Of course, since the heroes spend most of their time putting down creatures that are generally more dangerous and powerful than human beings, it's common to make these villains even more of a threat -- expect to see your heroes knocked about (physically or mentally) more than usual. Cannibals are a common choice, largely because they border on monster-level weirdness anyway and aren't quite as played out as the Serial Killer.

To help make the threat even more convincing, you can expect the writers to make these episodes darker than the usual fare, possibly through gruesome horror or psychological tension. The latter is particularly useful, since it can lead to the characters inflicting nasty violence on humans rather than monsters for once. If the heroes can't kill humans then expect a Karmic Death. And if the show is given to platitudes you can expect some 'the real monster is man' philosophising at the end -- even though the werewolves, zombies and demons that appear in all the other episodes make it clear that the real monsters are monsters.

The Mundanger may be part of a Scooby-Doo Hoax -- though with deadlier consequences than Scooby Doo would ordinarily encounter. Contrast How Unscientific, where a show set in an ordinary world features a seemingly real supernatural event in one episode.

Compare They Look Just Like Everyone Else.

Anime and Manga

 * Neon Genesis Evangelion, with the reveal in The Movie that the final Angel is "mankind", in the form of an All Your Base Are Belong to Us attack on the Geofront.
 * Jet Alone, a "malfunctioning" giant robot from a rival organization, also qualifies.
 * The most vicious, evil, and overall nasty Monsters Of The Week in Hell Teacher Nube are all human. Such as the masked serial killer who nearly murdered Kyoko and Hiroshi the con artists that did the same, the bank robbers who Nube humiliated with his supernatural skills until they shot at him with an Uzi, the kidnapper who took Nube's kids to a warehouse to kill them, the Mad Scientist who called down the Orochi upon the city...

Comic books

 * In the Hellblazer arc "The Family Man", something that's gruesomely killing families turns out to just be an old man with a big knife. And the mysterious beastie that's eating people outside the town of Doglick in "Good Intentions" turns out to just be a giant boar.
 * Notably, titular Family Man scares John more than most supernatural villains in the series.
 * An arc in Todd McFarlane's run on Spider-Man involved a series of gruesome slasher murders in the woods of Ontario. Spider-Man initially believed the murders to be the work of the monstrous Wendigo, but while investigating alongside Wolverine they learned that the murders were committed by a serial killer.
 * In a Hack Slash story it at first looked like old villain Father Wrath was back, but it turned out to be a non-powered copycat, whose neck Vlad nonchalantly snaps.

Film

 * Brotherhood of the Wolf centers around the true Urban Legend of a giant wolf who terrified France in the 1700's. Throughout the movie, it is believed that the wolf is a supernatural force.
 * Sociopathic Soldier Captain Vidal is by far the deadliest (and most disturbing) monster in Pan's Labyrinth.

Literature

 * Stephen King wrote Cujo with the declared goal of creating an entirely mundane horror story where the supernatural was absent.
 * Kim Newman's story "Where The Bodies Are Buried 3: Black And White And Red All Over". The other WTBAB stories were about a supernatural Serial Killer emerging from a Slasher Movie; this is a Ripped from the Headlines tale of tabloid hysteria and hypocrisy over such movies. (Yes, ultimately it's all down to Derek Leech, but he doesn't do anything supernatural to make it happen.)

Live Action TV

 * Supernatural has featured a couple of these: a Cannibal Clan in "The Benders" (named after a real-life cannibal family nicknamed The Bloody Benders) and feral children in "Family Remains".
 * The first season of Torchwood also featured a Cannibal Clan in "Countrycide".
 * Unsurpisingly, given its long run on TV, The X-Files featured a bunch of these. In "War Of The Coprophages", a cockroach infestation creates mass hysteria about an alien invasion, though the trope is subverted in that there really are robotic alien cockroaches, but they have nothing to do with the deaths in the episode. In "Grotesque", a serial killer believes he is possessed by a demon, though it is implied that this is more down to madness and obsession than genuine supernatural drama. "Home" sees Mulder and Scully tackling a trio of murderous inbred hicks. In both "Irresistible" and "Orison", a serial killer named Donnie Pfaster becomes obsessed with Scully; he is seen on a couple of occasions with a demonic visage, but this could just be his victims' fear warping their perception. In "Hell Money", an incinerated man turns out to be part of a grim organ-dealing gambling game. And in "Our Town", an entire town turns out to be a Cannibal Clan; although there is a slight supernatural edge to the piece -- eating human flesh, it turns out, provides you with extended life and youthfulness -- it's secondary to the actual threat.
 * "Quagmire", on the other hand, provides a Double Subversion: The prehistoric monster said to inhabit a lake where people have gone missing recently turns out to be a common crocodile. Mulder is...disappointed, but just as he leaves the scene the audience gets to see that there is an actual plesiosaur in the lake. Wether the entire episode is just one big Deconstruction or a vindication of Cryptozoology is anyone's guess.
 * Done to chilling effect in 'Paper Hearts' where Mulder conforts a child killer who claims to of taken mulder's sister. The episode ends ambiguously with Mulder's convictions of her alien abduction shaken.
 * In the Charmed episode "Sight Unseen", a stalker turns out to be a human woman and not a demonic threat after all.
 * Although several of the earliest Doctor Who episodes had nothing science-fictional beyond The Doctor and his time machine, the only post-'60s story to feature nothing fantastical at all (apart from the presence of the main characters) was the two-part murder mystery "Black Orchid".
 * The new series episode "Midnight", while it still had an Eldritch Abomination alien menace, was far more focused on the people of the episode, who grow more and more paranoid before nearly stretching to murder. What makes it Mundanger is that while the alien presence does manipulate people somewhat, the humans are the ones who are the real threat. A fantastic example of mob mentality. The episode is milked for all the Psychological Horror and Humans Are Bastards it can, a disturbing contrast to the usually quite idealistic program.
 * Buffy the Vampire Slayer had a minor villain in the school's lunch lady, who tried to kill everyone in the school by dumping rat poison in their food. Buffy had accidentally acquired telepathy at the time and heard her thoughts, allowing Buffy to stop her.
 * The gang never actually thought they were dealing with something supernatural though, just a disgruntled teacher or student who planned on shooting up the place (Willow even prints off FBI mass murderer profiles so they can look for someone who displays what they list).
 * Although, this kind of thing happens in Sunnydale more often than other places because the Hellmouth sends out evil-crazy vibes.
 * Played across the entire sixth season of the show with Buffy being ineffectually challenged by the Trio, a group of human 'villains' (in the loosest sense of the word) with almost no actual powers between them. Inverted initially as, rather than being more challenging or horrifying than normal monsters, they actually spend most of the season being pretty useless nemeses,  "Who's the real monster?" and all that...
 * Also,  died of natural causes, completely unrelated to the fact that   was the Slayer..
 * The first-season episode "Nightmares" featured people's nightmares coming to life, a supernatural phenomenon which was inadvertently caused by the astral projections of a boy in a coma. However, the boy was put into the coma by the true villain of the episode: his Little League coach, who blamed him for the team's loss.
 * The occasional episode of Friday the 13th: The Series, the most well-known one probably being "The Long Road"; on the way back to Curious Goods after getting a new artifact, Micki and Johnny are taken captive by inbred brothers, who murder people so they can stuff the corpses and put them on display.
 * Variation in Fringe episode "Northwest Passage". The killer is just a lone killer whose methods happen to look a lot like something done by the conspiracy, to an absurdly specific degree (he takes pieces of the temporal lobe of the brain). It would fit except that the conspiracy is not actually supernatural.
 * Criminal Minds had some variations on this: the villains are usually serial or spree killers, but one episode had a guy disguise a pragmatic, money-motivated murder by committing other murders, so that it looked the work of a serial killer. Another one had a killer who committed a double murder and tried to disguise it as the work of a cult.
 * In Highlander Duncan is able to protect Tess and Ritchie from many Immortals who wanted to hurt them to get to him. However,

Tabletop RPG

 * Call of Cthulhu (tabletop game) normally involves players going up against the creatures and cultists of the Cthulhu Mythos. One adventure, "Westchester" House, was about a "haunted house" where the hauntings were strictly human created, with nothing from the Mythos involved.
 * Although the Ravenloft setting is best known for its vampires, werebeasts, mad scientists and other horror staples, it's also home to a number of Mundanger human killers. One of the Core's bloodiest darklords, the tyrant Vlad Drakov, is an ordinary fighter whose only supernatural quality is an enhanced resistance to magic. The (non-darklord) ruler of Nova Vaasa, Prince Othmar, is likewise a normal human villain.
 * Tends to happen to you in The World of Darkness if you survive long enough.

Video Games

 * The Chosen One comes across all kinds of strangeness in Fallout 2, from ghosts to super-mutants to intelligent scorpions. But when (s)he investigates giant scorpion monsters that are kidnapping cattle at night, they turn out to just be a pair of cattle rustlers.
 * Technically, this was so early in the game, that all the supernatural threads should not have shown up yet. So it probably wouldn't count.

Western Animation

 * The Sam and Max Freelance Police cartoon pitted them against their usual selection of crazy monsters, giant robots, aliens, and so forth - as well as against their loserly human Loony Fan, Lorne. He is easily the most fondly-regarded villain in the show.
 * In Batman: The Animated Series, there's often a normal human gangster or corrupt boss behind the villainy of the week. In a lot of cases, they're even more evil than the costumed villains are, like Boyle, the guy who created Mister Freeze by callously trying to pull the plug on his wife.

Real Life

 * Despite news media and realistic fiction focusing on threats of war, gang crime, terrorism, starvation, and serial killers, most Americans will die of heart disease or cancer, which each claim more lives than the sum of the next three causes of death, cerebrovascular diseases (such as stroke), chronic lower respiratory diseases, and accidents. Suicide was 11th, and homicide was 15th, with every other slot filled by a disease or condition.
 * One episode of This American Life, The Giant Pool of Money, included a story of a Marine, returned from Iraq, trying to catch up with his mortgage payments after the housing bubble collapsed and his interest rates went through the roof. In the followup episode sixteen months later, This American Life reported that the Marine ultimately got the loan refinanced to a point that he could keep up, but that it ended up being even more stressful than being deployed in the military.
 * In Europe, many of the purported attacks of bears, wolves and lynxes against livestock are actually the work of feral dogs. Farmers are obviously more inclined to blame the former since that guarantees a government compensation for their losses, whereas the latter usually doesn't.