If all you know about Higurashi is the cover, you will be very surprised by the beginning of the first episode, and troubled by the vaguely sinister opening credits, but the bulk of the episode will seem like a typical Slice of Life comedy featuring a boy who has recently moved to a cheerful small town and befriended four female classmates... but before the first episode is over, the boy has realized he's living in a Town with a Dark Secret, and everything starts spinning out of control, and by the end of the fourth episode, half of the main characters have died gruesomely, including the Decoy Protagonist, and you realize the beginning of the first episode showed you that this was going to happen. And then the fifth episode begins with one of the younger main characters stabbing herself to death with a huge knife for no apparent reason, and after the opening credits, it's back to light-hearted comedy.
The head-stab suicide is accompanied by another cast member (who had been shown to be a good friend of hers) standing there, smiling and eventually laughing creepily.
Naru Taru seems at first like a cute little story about a girl and her pet dragon-thingy. Then it starts going horribly wrong...
This seems to be typical of Mohiro Kitoh's work. Bokurano is an initially charming Humongous Mecha show with child protagonists. All of those kids are going down, and you'd need to use factorials to calculate how many universes get completely destroyed over the course of the series. And Wings Of Vendemiaire is about charming little living puppets going through horrible, horrible things.
Tenshi Ni Narumon: Painfully sweet, cutesy, silly and looks like it was made for some toy store line. By the end of the series, the darkness, creepiness and mindfuckiness will make many cringe. And there's a pretty explicit yaoi innuendo going on there too.
Dai Mahou Touge (or Magical Witch Puni-chan) features a magical princess sent to a high school in the human world as part of her princess training. She has a cute talking animal side-kick, a magic wand, and the ability to animate vegetables into evil minions (or to jump into whatever she's cooking). Oh, and did we mention that this adorable little girl also wrestles opponents with the intention of breaking bones? Well, as Puni says, "submission is a princess's way!"
Also notable because her animal sidekick, who was forced to be her pet after she kicked the tar out of him, constantly wants to kill her. And within the magical kingdom not everything is so magical; for instance, an army of slaves is used to move trains. None of this is ever shocking to Puni, but her normal human companions are often either stunned or frightened.
School Days starts out as a cute teenage love story, but then becomes disturbingly dark towards the last few episodes.
Momotaro's Divine Sea Warriors starts out as a cutesy film with adorable animals, but once the title character shows up it turns into disturbing war propaganda.
Fan Works
The My Little Pony: Friendship Is Magic fangame Story of the Blanks starts out as a nice, interesting little game starring cute little Apple Bloom finding a town full of blank flank ponies in the Everfree Forest. Then you find a skeleton in the fire place. And that's just how the scary part STARTS.
In the Pony POV Serieschapter "Playtime", while still overall having a somewhat creepy feel due to the fact Fluttershy's Superpowered Evil Side Princess Gaia is taking over Equestria, is mostly lighthearted, focusing on the Mane Cast talking with Princess Gaia as if she were simply Fluttershy and the foals that Ponyville has been turned into just playing around. Then Applejack realizes Princess Gaia's eyes have neither lies nor truth in them. Princess Gaia's facade shatters, revealing her true form, NightmareWhisper.
Film
The Brave Little Toaster. Just to name a few...right off the bat we have Air Conditioner raging himself to death, then there's the Nightmare Sequence involving a Monster Clown, then, near the end of the film, there's a musical number in which anthropomorphic cars are being crushed to death.
ALL of Don Bluth's films have examples of this trope. For instance:
An American Tail: Mostly fine, but Fievel's separation from his parents, whose hope in finding him wears thin quick, is rather sad. Then at the end, the Giant Mouse Of Minsk.
The Land Before Time: Cute movie about dinosaurs... The mother of one of them is killed by a T Rex who in turn, is drowned in a hot spring.
Meet the Robinsons. The first 3/4s of the movie are full of colors, gleeful wackiness and literal bubbles. Then, just after what appears to be a daring rescue, the robot friend gets speared through the chest and dies, the main character's best friend /future son disintegrates into nothingness, the world is plummeted into despotic despair and slavery, and the main character's family, being controlled via electronic hats that cover their faces, starts trying to kill him by beating him off a very high fall. All in the space of about two minutes.
Willy Wonka and the Chocolate Factory. First everything is nice, with the golden tickets and the chocolate river... then there's the boat ride... Then again, it is based on a Roald Dahl book.
The same goes for Matilda, with its freaky wide-angle closeups and uncannily cartoonish torture/violence.
Literature
The fantasy novel series Fablehaven. Despite its bright and cheerful kid-aimed covers, it gets smacked upside the head by our friend Cerebus remarkably quickly. Each book is darker than the last, but the covers remain cute and inviting.
Olivia Kidney has a title protagonist who appears on the cover as an adorable, cartoony little girl. She talks to ghosts all the time, including her beloved dead brother. It isn't quite Tideland, but brrrr.
And that's just the tip of the iceberg as far as all the creepy. On top of it, there's the ever-present implication that everything that's happening might just be a product of an extremely overactive imagination. Or maybe it's all real. They never really let you know for sure, and it's not clear which would actually be worse.
Redwall starts out seeming like a sweet little Low Fantasy series about cute fuzzy animals in a pretty woodland setting. Then some pretty damn horrifying things happen to them. It says quite a bit about this children's series that it was the first work of literature to get its own Complete Monster page.
When unhappy or unpleasant truths intrude on the relatively peaceful lives of characters in the Aunt Dimity cozy mystery series, they often do so in this way. Open a box expecting to see maybe an heirloom—find fully dressed human remains. Heave a sigh a relief that the "murder" turns out to have been an accident—well, that's jolly nice, but your hero has just had a flashback on a real murder he witnessed and is near catatonic. Similar mood dynamics occur when solutions are given and the culprit turns out to be obsessed and/or insane.
Harry Potter and the Chamber of Secrets. Things get insanely creepy when Harry starts hearing whispering in the walls. You could also say this about the entire series, starting off as a whimsical children's series and then taking a very dark turn in book 4.
Dr. Seuss wrote Thidwick the Big-Hearted Moose, which ends in Thidwick's unwanted guests being killed and taxidermized.
Video Games
Myst IV's Serenia - a lovely world of glowing butterflies, literal stone forests, and magic, memory storing lotuses. And then comes the Spirit Ceremony and the Big Bad's plan for Yeesha.
A lot of Myst has this. Special mention goes to the brothers' rooms in the first game.
In Psychonauts, Milla's brain seems to contain nothing but fun and parties. However, if you manage to find a slightly out-of-the-way area, you discover that she used to work at an Orphanage of Love... which was destroyed and all the orphans killed in a house fire and continue to haunt her nightmares, crying out things like, "Save us, Milla," and "Milla, why won't you help us?"
Ecco The Dolphin - A game starring a cute dolphin who has lost his family to extremely hungry aliens. He must go back and forth through time, battling past sharks, jellyfish, enormous spiders, and trilobites, braving eerie music and disturbingly solemn, frightened fellow dolphins, to rescue them and the Earth. Later locales include a meat grinder scrolling level where you're continually molested by terrifying masses of chitin and blobs of acidic green goo, and slime tubes in the sky that cause you to plummet five miles into roadpizza if you mess up and fall out. Rated K-A E for Everyone!
The final game in the series even has you swim inside the body of a giant alien and attack its beating heart.
Aquaria follows a similar premise as Ecco, only this time the protagonist is a fish girl. She finds herself utterly alone and starts to explore the ruined civilizations, and eventually meets their crazy gods and the Big Bad who made them. And yes, just like in Ecco, there are Womb Levels in Aquaria.
Indie game Eversion plays with this. And by "plays with this," we mean it's the premise of the whole game. Although if you know who H. P. Lovecraft is, the opening screen serves as a warning about that, as the game starts off by quoting him.
Kingdom Hearts. Having the Disney Label on it and trailers featuring tons and tons of our beloved characters from Disney Animated Canon made it seem like this game was intended for children ages 6 and up. Then, the Final Fantasy elements showed up and the OTHER feeling Disney is famous for. Not only that, but the game added its own Nightmare Fuel in form of the Heartless; creatures who, contrary to what their names imply, don't lack hearts; they ARE hearts corrupted by the darkness. These little monsters have one goal in mind: To corrupt all hearts that there are, and the hearts of the worlds they haunt are especially sought after. You know that the cheerful game those trailers announced isn't that cheerful when you see an innocent NPC being turned into one of those monsters as his face freezes in a fearful expression, and that's toward the beginning of the whole game.
The sequel, Kingdom Hearts 2, introduced the Nobodies, monsters made from the body and soul that the darkness consumed heart leaves behind. The only thing these creatures strive for are to be whole again, to find their Heartless and reunite with it. That's a sympathetic cause, right? Well, the problem is that since a Nobody is, due to its lack of heart, beraved of emotions, they are by their very nature sociopaths that will stop at nothing to succeed. As if all of that wasn't creepy enough, due to being "hollow", the lower ranked Nobodies move as if they were husks of fabric or paper filled with air.
Chain of Memories features the Sora slowly being brainwashed into forgetting every single person who really cares about him in favor of a girl he's never even met in an attempt to make him into the villains' puppet.
"Miss World '96 Nude" is infamous as a horror game posing as a porn game. The gameplay consists of removing the obstructions that block your view of pictures of scantily-clad women. Take too long, and the image "turns" - the woman turns into a hideous monster as the gameplay gets harder. It's the ultimate in Fetish Retardant, and freaky.
Once the game turned into its horror side, it started to play a short loop of "YOUR BODY, MY BODY, EVERYBODY MOVE YOUR BODY!" You'd think that'd be narmy, but somehow, it can come off as even more unsettling due to Soundtrack Dissonance.
40 Winks was intended to be a psuedo-nightmare played in a way that was positively adorable. Even though the "scary" aspects of the game were mild, the completely innocuous music and eerie simlish were complete Nightmare Fuel.
Okami: Most locations are vibrant with colorful characters, bright music, and thick-line artwork reminiscent of a Japanese scroll painting. The surprise comes mostly from two locations: the Sunken Ship and Yoshpet forest. They are creepy and quite jarring in contrast to the rest of the world. And there's also the Body Horror parts where you go inside someone's body, which is as nasty-looking as you'd expect (particularly the stomach acid dripping from the water dragon).
The cursed zones are pretty creepy, too. Two-layered, as well: the first impression you get when you see your first cursed zone is that it's made the land desolate and barren. Then when you actually try to walk directly across it, everything becomes dark, the music becomes incredibly ominous, and you die if you stay there too long.
Okamiden even more so. The rating is lower, the player character is painfully cute, and then you see tank-sized skeletons made of the bones of murder victims and a Player Punch to end all Player Punches.
Free Realms: Merry Vale, the 24-hour party place where everyone has fun! ...too much fun notice that their kids are getting abducted.
The Elder Scrolls IV Oblivion has Camoran's Paradise. Upon arriving there, one might be wondering why a pocket dimension created by the Big Bad is a idyllic forest chock full of Scenery Porn, which would actually be relaxing if it wasn't full of tough enemies. Then you get to the caverns under the whole place, and find out that it's where Camoran tortures his own followers for all eternity (there's one section where you see someone locked in a gibbet hanging over a river of lava. And yes, you can lower it), since all of them have been made immortal so that the torture lasts forever. How calming do you find it now?
Judging by how many Game Mods let you go back and explore, very.
Hatoful Boyfriend: A very, very silly game about dating pigeons. At least until the backstory reveals itself.
Super Filly Adventure is a fun little My Little Pony: Friendship Is Magic fangame that has you attend one of Pinkie Pie's parties and save Ponyville with your flank-kicking combat abilities. Even the "bad" ending where you become so obsessed with baking muffins that Ponyville burns down is far more silly than disturbing. Then you discover the secret "Blank Flank" ending, which is accessible only if you play the game at a very late hour...
Theater
Pippin- The songs, the poster, even the characters will try and convince you this is a musical comedy "coming of age" tale. It's not.
Nintendo
These guys and their development teams may well be the reigning champions of this trope. They have a family-friendly, even kiddy image, but quite a few of their games are clearly intended for brave kids.
The Legend of Zelda - Primarily E-rated games. E-rated games that include, among other things, a temple dedicated to death, zombies that are either trying to eat or rape the protagonist, and the apocalypse. Probably the two most blatant examples are Ocarina of Time and Majora's Mask. The ESRB seems to have caught on by the time of Twilight Princess, but even so...Twilight Princess's T rating was probably just because of the darker graphical style, since Skyward Sword has an E10+ rating despite not being too much more lighthearted.
And just when we thought Nintendo made The Legend of Zelda: The Wind Waker specifically to be absolutely and completely harmless, along comes Link stabbing Ganondorf in the brain. There are reports of players who dropped their controllers the moment this happened. Also, when you consider how incredibly dark the backstory of this game is, it suddenly seems like the game's happy atmosphere was taunting the past and the people of ancient Hyrule, much like how the happy music in Clock Town seemed to taunt the people who stayed oblivious to their impending doom, only more subtle.
Skyward Sword has its moments too, most notably in the Ancient Cistern. At first it seems a bright and pleasant place with lots of clear water and green and gold scenery. And then you descend into the lower parts...Which culminates in Link climbing a rope to escape only to find that a horde of zombies is also climbing that same rope... and then you learn that to continue, you have to go back down that same rope...
Skyward Sword has also Ghirahim threatening that Link's 'punishment' will be so severe he'll deafen himself with the shrill sound of his own screams. Rated E-10!
Pikmin - More E-rated goodness that involves sending adorable little carrot-creatures to near-certain doom against beasts many times their size. In the first game Olimar does this to survive. In the second, it's all about getting treasure. Some of the more bizarre enemies in the game include a frog-shaped embodiment of poison and the moaning intangible Eldritch Abomination known as the Waterwraith. Olimar actually does wonder:
Would this have been a peaceful planet had I never come? No... Surely the Pikmin lived like this before my arrival. They MUST have.
Pokemon Mystery Dungeon: Explorers of Time/Darkness/Sky - A game which starts out with cute little monsters, in particular the nervous partner character, learning how to be good explorers. Starts getting a little uncomfortable when a child is abducted by a criminal, gets out and out distressing when you get dragged into the future, and the game really doesn't let up on the creepy for the rest of the story. You and your partner are encouraged to commit suicide at one point! Yep, E-rated.
The mainline Pokemon games themselves have started to get in on the fun, especially with the creepier Pokedex entries - see entries like Yamask and Shedinja's just for starters. Then there's Pokemon Black And White... would you like a nice cup of horrific psychological abuse of a child with your cute monster battling?
Pokemon Diamond, Pearl, and Platinum has pretty creepy moments as well. In all three games there are fun adventures, cute friendly monsters, and an evil cult that wants to destroy all of existence so their leader can reshape the world in his image and become God. Oh, and did we mention that Platinum contains an extra segment in the main storyline where you end up in a parallel universe and face off against a massive Eldritch Abomination of a dragon?
This trope has shown up in just about every single Pokemon game, starting with Gen 1's Lavender Town. Between the background music being more unnerving than it should be possible for any 8-bit song to be and the sudden revelation that Pokemon can actually die, there's the entire origin story of Cubone/Marowak and its/their skull-helmet thing.
EarthBound. 99% Cartoon-y goofiness, 1% incomprehensible cosmic horror. And it's the last 1%, too.
Mario gives Link a run for his money throughout the Paper Mario games, which end up a lot like Earthbound with the subject of murder (Though he was actually just unconscious) dealt with in the first game; an Eldritch Abomination appearing as the final boss of the second; and the third's plot being explicitly that reality itself was being destroyed, later revealed that this includes The Afterlife. Also notable is Super Mario Galaxy which is hours of compressed sheer joy — and then the universe is annihilated in a supermassive black hole.
The Kirby games feature a very cute eponymous protagonist, colorful and fanciful levels, and lots of frightening bosses. One of them is actually named Nightmare, and he's not even the scariest by the long shot! Perhaps most infamous among them are Marx Soul, especially his death sequence, and Zero, mostly for his Clipped Wing Angel transformation. Kirby 64 continues the tradition; while Dark Matter and especially the possession scenes are at least unsettling, they still don't provide any good warning for the jarring final boss. It took Kirbys Return To Dreamland to finally jack up the age rating to E10+, and the games mentioned earlier only got an E10+ rating when the Kirby Dream Collection was announced. For reference, that's over a decade since their original releases.
Magical Starsign. The game is saccharine and kid-friendly through-and-through until Semolina commits suicide through a carnivorous flower before you get a chance to protest, and becomes one of your Cosmic Keystones. Later, you get to learn the fate of Applepie, and/or every human in the universe once the robots start to run out of fuel: robots turn humans into "gummies", their battery cores. And that's ignoring the star-devouring abomination that is the Final Boss...
Monster Pulse: It's all fun and games until someone gets their head bashed in with a rock. Although the entire premise of "organs being ripped from the body and turning into giant monsters" might also fit the bill.
Quite a few series in the Slender Man Mythos use this trope. Several start out as ordinary video or web blogs that seem perfectly normal for the first few entries, only for things to go creepy when Tall Dark And Faceless pops up.
"Counting Song" by Adam Buxton starts off as an ordinary, kids sound about numbers set in a Sugar Bowl. It quickly turns into a lament about how terrible the adult world is, all while keeping its cute look (albeit a lot more grey).
Bob the Dancing Camel. A goofy poem about a circus camel that runs away from his abusive master and joins up with a singing rat to become famous but then said master gets his revenge.
Can Your Pet? is a cute little virtual pet game. At first.
For that matter, the episode Lesson Zero qualifies. It seems at first to just be a slightly more meta episode than usual. Keep in mind, however, that this spent a brief amount of time as the only episode of any television series that had its own Nightmare Fuel page on this wiki.
Fans have speculated that Twilight's breakdown in that episode was caused by Discord's actions towards her. True or not, it arguably makes the episode and Discord just that much creepier.
In "Hearth's Warming Eve'', the play starts out somewhat serious, but still not that bad. Cue Eldritch Abominations attempting to freeze the entire world. Keep in mind that the play is based directly off of the history of Equestria's founding, so it most certainly did happen in real life.
"Baby Cakes": Aww, it's a babysitting episode, how kyuuute! Wait, where'd they go, and what's with the sinister music?
"A Canterlot Wedding:" Twilight Sparkle has apparently messed up big time, and she's sad, but it looks like Cadance and she will make up. Then HOLY HORSEAPPLES "Cadence" is evil!
Worth mentioning is that unlike with Discord, the previews gave absolutely no indication there'd even be a villain, let alone an absolutely terrifying and truly monstrous one at that.
The Powerpuff Girls has the infamous episode "Speed Demon." The Powerpuffs decide to race home from school...and go so fast that they warp into a strange, warped and destroyed version of Townsville. There, they see that the townspeople have gone insane. The Mayor is dead and Miss Bellum blames herself. The Professor (their father figure) is obsessed with futilely recreating his girls (and attacks the real things when he thinks they're hallucinations). And their kind-hearted schoolteacher just stands in place, right where she was waving goodbye to the Powerpuff Girls before they vanished for over 50 years. It's at this point that the already-creepy villain known only as Him shows up and reveals his true form, informing the girls that because they weren't around to save the day, this nightmare world is all their fault. They manage to go back in time so they can stop the Bad Future by being there to save the day, but the Fridge Horror of the episode is palpable.
Rango starts out as a fairly light-hearted Western for kids, with only a few sinister villains to occasionally darken the skies. Then Rattlesnake Jake shows up, and introduces us to a new meaning of fear.
In the scene when the animals are underground, following the bank robber's path. In the background as they are crossing a bridge, a giant eye opens up and watches them go by, easily bigger then most of the characters in the movie, it's never explained.
The Marvelous Misadventures of Flapjack, which was created by some of the same crew, is also like this. One episode showed what appeared to be a cute child, turn out to be a creepy looking older guy.
As the picture shows, a short called "The Snowman". Starts off really cutesy, with a human and his animal friends building a snowman and dancing around, but all hell breaks loose when the snowman comes to life and goes on a rampage.
"Circus Capers", a cartoon that blatantly plagiarizes the designs of Mickey and Minnie Mouse, starts off pretty generic and happy...then Minnie is revealed to be cheating on mickey with the obvious villain of the cartoon. Mickey discovers this, and almost instantly becomes fat and depressed. He begins to sing "laugh clown, laugh" with a horridly wrinkled and disfigured face. In response, Minnie realizes her horrid mistake, attempting to return to mickey. He rejects her, and, out of nowhere, HIS FACE COMES OFF OF HIS BODY, ENLARGES, WINKS AT THE SCREEN, AND INFLATES, COVERING THE ENTIRE SCREEN WITH THE BLACKNESS OF HIS NOSE. What follows in every salvaged copy of the cartoon is a quick cut to a bunch of happy dancing ducks from "the booze hangs high", an actual warner bros cartoon. The sudden, unpredictable shift may get more jumps than anything, but perhaps the most horrifying thing of all is that we'll never know exactly how "Circus Capers" actually ends and what mutated wrinkled obese clown mickey does.
Actually, this should surprise no one, since it's apparently from the same studio that gave us the page image.
The original My Little Pony pilot. Starts off with rainbow ponies playing around and a cute song, and then suddenly switches to ponies being kidnapped. It only gets worse after that with the ponies being turned into monsters, and the villain threatening to decapitate a child character.
