Saint Seiya. Blatantly obvious in the anime, with the addition of movies and a Filler arc. To the point where greek gods, Norse gods, Buddha, and friggin' Satan fought Seiya and Co. And the universe was created by Big Bang, so it's possible that Athena reincarnated as a Raptor.
Mazinger Z and Great Mazinger: Dr. Hell joined an archaeological expedition to the Greek island of Bardos, thinking maybe several ancient legends told that island was defended by an army of mechanical giants were true. Unfortunately to everybody else, he was right. Classical Mythology plays an increasingly important role in each retelling of the series, until the point of Greek gods start showing up and Great MazingerBig Bad is revealed being Hades in Shin Mazinger.
Shaman King also does mention about all prophets/chosen people in different cultures as being Shaman Kings from previous tournaments, although they only imply that with the most known ones, Jesus and Buddha. Some spirits used by shamans seem to be portrayed as Gods, too, like Shamash, and the Sphynx.
Devilman has Akira's friend Ryo tell him that Demons once roamed the Earth before being frozen in the arctic while humans dominated the planet. He also mentions that some demons got free, and could be the true causes of monster myths like Wolfmen, Dracula, and Ogres.
Guyver suggests that the zoanoids changing between human and monster forms is the origin of myths like werewolves and vampires.
Only three legends are ever mentioned in Fullmetal Alchemist, two of which turn out to be true. The Xingese legend of the Western Sage is about Ed and Al's father, while the Amestrian legend of the Eastern Sage is about "Father," the Big Bad of the series. The third myth is presented when Ed compares the circumstances that caused his own amputations to the story of Icarus (of Greek Mythology) flying into the sun and getting burned. Whether the Icarus myth is true in the FMA world is never clarified.
The story begins with Shuurei telling her students the story of their country's founding, ending it by saying that according to legend, the eight immortal sages who helped the first emperor found Saiunkoku are still alive in secret among the people. This is absolutely true, and Shuurei goes on to become personally (albeit unwittingly) acquainted with several of them.
A little later in the first arc, Shuurei begins to tell Ryuuki the story of the Rose Princess and how she married a mortal man. This story is not only also true, it's the story of her parents' marriage.
High School D×D has alot of mythologies existing in this story, Ars Goetia being the most prominent example. Norse Mythology, Hindu Mythology, Greek Mythology, it's all here though apparently there's also one more mythology that even the other gods of said mythology don't know about. The name of said mythology? There is a breast god in this series.
Subverted in Dragon Ball when Roshi tells Krillin and Launch the origin myth of the Dragon Balls. The legend he tells is more realistically played: parts of it are true or are based on reality, but the portrayal of it is false.
Double Subverted in the Big Finish Doctor Who adventure Neverland. The Doctor and the Time Lords head into a universe of Anti-Time to fix Charley's paradox, but it is revealed that the Time Lords have other motivations for heading there. It is revealed that legends on multiple planets speak of the great Time Lord Rassilon heading into the empire of Zagreus (the Anti-Verse) to face the beast himself. The legends seem to be true when they find the casket of Rassilon, which is given to them by the denizens of the Anti-Verse. However, it is revealed that the legends of Rassilon and Zagreus were planted by these so-called Neverpeople in order to lure the Time Lords into bringing the casket, which actually contains enough Anti-Time to destroy the universe. However, when the Doctor absorbs the Anti-Time into himself and the TARDIS to save the universe, a creature of Anti-Time is created inside him. It decides to take it's name from a creature of legend.
Maureen Raven: Oh, for God's sake, the I Ching is true? Is there anything that isn't true?
This was the original premise of Marvel Comics' Eternals, before they were shoehorned into the mainstream Marvel Universe. The Jack Kirby series had these beings and their enemies the Deviants, mistaken for gods and monsters and inspiring all of humanity's myths, legends and ancient religions. When they got switched to the Fantasy Kitchen Sink of the MU, they were relegated to having merely been mistaken for actually-existing gods.
In the Marvel Universe you have Thor and Hercules able to work together. Heck, in the Marvel Universe, ALL pantheons are real. Their leaders have the occasional meeting, for crises such as, say; Skrull invasions? In fact, the approach taken by Marvel seems to be that EVERYTHING they have ever published—not just the superhero comics, but horror, science fiction, romance, western, humor etc. are ALL TRUE and part of the same setting—details to be worked out on a case by case basis. Yes, even Howard the Duck.
In The DCU, even if you just look at the Marvel Family, you've got Captain Marvel whose powers come from Solomon and a selection of Greek and Roman figures, as well as his rival, Black Adam who gets HIS powers from the Egyptian pantheon. Both collections of myths spell out "SHAZAM", so they both have the same magic transformation word.
The overarching plot of both Fables and Jack of Fables is, of course, that all fictional characters really exist and are living in New York. Jack of Fables introduces characters that represent literary devices, the most amusing of which is probably the Pathetic Fallacy.
DC Comics is like Marvel in its 'everything we published counts' approach, though with a few more notable exceptions- which of their Vertigo line of comics stories count and which don't isn't terribly clear, for example. Or at least, it exists in some form (like Batman of Zur-En-Arrh being a psychological construct, a 'backup' for if Batman's mind should be otherwise broken.) These days, with a Crisis CrossoverCosmic Retconning out everything the current writer doesn't like at least once ever other year, it's hard to know what of the previous month's comics counted at any given time, let alone one from fifty years ago. However, we do have the approach to myth and legend as the trope describes: Greek gods? Totally exist. King Arthur? Ditto. Ghosts, The Legions of Hell, things called demons that aren'tThe Legions of Hell, vampires, Biblical figures? Even characters who don't primarily deal with the supernatural have had multiple run-ins with all of the above. We have main characters in most of those categories, in fact.
Ganesh: The Earth is so old, and home to so many strange things, that there is hardly an inch of ground that was never home to a shrine, or a god, or a battle, or some magical oddity. Even under the ground, you yourself have said, there are old gods, old prophecies, old lost things. It is not odd that this bound god should be here, in this place. If anything, it is odd that we are not constantly hip-deep in such magical echoes of the past.
Neil Gaiman's The Sandman. Where we learn that not only are all myths true, they themselves take a back seat to an even deeper and all-encompassing group of seven siblings known as The Endless, who embody seven big forces powering all the mythos throughout the entire universe.
iZombie. It's got the titular zombie, a ghost, a group of vampires, and a were-terrier. And this is all in the first two issues.
The CVO: Covert Vampiric Operations series has this at its core. The titular squad of vampires fights all sorts of supernatural threats. In fact, the only major human member of CVO is their boss Overmars, whose orders the vampires follow without question (most of the time). Overmars's Number Two is an erudite demon named Nikodemus (who looks a little weird, being all red with large horns while wearing a suit). Their scientific expert is a nerdy zombie (who hasn't lost his mind or gained a taste for human flesh). In later issues, they get two more operatives, one of which is a human Genius Bruiser the size of a defensive lineman and a Japanese katana-wielding girl who can turn into a snake-like creature complete with Sssssnake Talk. Their normal enemies include everything from zombies and demons to aliens and Eldritch Abominations. They also have Magitek called Artillica, which appears to be the focus of many issues. And that's just a sampling.
In "Sleeping Beauty", when the prince comes and asks after the castle, he gets a whole slew of false answers; although one old man does know the truth, it's not the popular one.
Everyone answered according as they had heard. Some said that it was a ruinous old castle, haunted by spirits. Others, That all the sorcerers and witches of the country kept there their Sabbath or night's meeting. The common opinion was: That an ogre lived there, and that he carried thither all the little children he could catch, that he might eat them up at his leisure, without anybody being able to follow him, as having himself only the power to pass through the wood.
Cosmic Warriors, an AU Sailor Moon retelling, the author starts the story off with the first villain Usagi faces being a reincarnated hero from Irish mythology, Diarmuid Ua Duibhne.
The Harry Potter fanfic One World, dramatically expands the list of mythical creatures that exist in the Harry Potter universe due to research on various myths. Thus far Hogwarts has a professor that's a Drow, Selkies and Knuckers in Black Lake, Dungeons & Dragons was invented by a former Cursebreaker that lost his magic, and Voldemort has been shown to negotiate with devils and demons.
Jerry (reading from a list of questions): "'Is Jesus Christ the son of God?'"
God: "Jesus was my son. Buddha was my son. Mohammed, Moses, you, the man who said there was no room at the inn, was my son."
The TV-Movie series, The Librarian completely runs on this, especially since it's the Librarian's job to find and store all the world's legendary and mystical items in a hidden underground room in the Metropolitan Public Library so that they're be safe and won't be used for evil purposes. The room literally has everything: Pandora's Box, The Goose That Laid the Golden Egg, H.G. Wells' time machine, Excalibur, etc.
Pirates of the Caribbean, which features Aztec and Greek (Calypso) gods, plus Davy Jones, the Fountain of Youth and working voodoo.
In Ever After, we find out that the legend of Cinderella stems from a true story of the narrator's great-great-grandmother.
The Alteriens of Alterien are actually the reason humans created the legend of elves and fairies.
In Astral Dawn, Caspian learns the old myths and legends were inspired from beyond the mortal plane by the actual beings those myths and legends were based on.
In Erika Griffin's novel, The One Who Waited, Alice ponders this during the course of the story, as she comes to realize that there are such things as Boogeymen and wonders if other monsters might exist as well.
Stephen Marley's Chia Black Dragon trilogy Sorceress, Spirit Mirror, and Mortal Mask, take place in 2nd century China, but there also appear Indian Buddhists, ancient Egyptians (in the back story) and a few Christians. It is suggested that the mythologies and afterlives of all four religions (Chinese, Buddhist, Egyptian and Christian) all exist. In addition to the Stephen Marley's own original myths and creatures, of course
This is the whole point of Neil Gaiman's novel American Gods, in which every god/spirit/devil/etc. that mankind has ever dreamed up are still around, mostly living like normal folks. (For instance, Thoth and Anubis run a funeral parlor.)
In K.A. Applegate's Everworld series, every god from every mythology gets together, and they create a parallel universe where they all rule. Complete with mythical creatures in addition to humans and mundane wildlife. This causes some problems when every god has an extensive cult, and they're all militant. Kill the heretic for worshiping Aphrodite and not Quetzalcoatl! Furthermore, several alien gods from other universes decide to crash the party, including the god-eating god Ka Anor of the Hetwan.
Percy Jackson and the Olympians, the Greek gods are real: features of Greek myths move around depending on where the center of Western civilization is. Olympus is on top of the Empire State Building; the Underworld is in Los Angeles.
After The Heroes of Olympus concludes, Riordan will be writing a series based on Norse mythology.
Robert A. Heinlein's later novels, beginning with the novel The Number of the Beast (though he used the concept almost 40 years earlier in his short story Elsewhen), deal with the World As Myth, and expand it to the multiverse. In his multiverse, All Stories Are True and Exist, somewhere — and if you've read the stories, it's possible to visit the universe in which the story takes place. He shows this by having his four protagonists visit several universes, albeit unknowing. A side effect of this is that all worlds are part of a story, somewhere... and that anyone who writes a story has become the literal God of the universe the story creates.
Harry Potter's world uses this trope, much to Uncle Vernon's dismay.
Children's author Robin Jarvis loves this trope. The ending of the Wyrd Museum series features the deaths of the Nornir by the Spear of Antioch, as well as the ice giants being finally defeated by the Eye of Balor on a spinning weathercock.
This was thousands of years ago, remember. Humankind and animalkind were closer then—our years in the Garden together were not so long ago. And our kind, the lycanthropes, were the bridge between the two...It saddens me that the tribes in this country do not tell the old tales to one another. If we gathered to tell stories and drink more, there would not be so much fighting, yes?
In Jasper Fforde's Thursday Next and Nursery Crime series all fictional characters are real(ish) and exist in a parallel universe called the Well of Stories. Fictional characters do have a few traits that differentiate them from "real" people (it's complicated), but in the Bookworld all stories are true.
In the universe of Christopher Moore's books the First Nation Trickster God Coyote is the younger brother of the Egyptian deity Anubis, Jesus plays poker with an upstart Cargo Cult deity and there are vampires, djinn and angels, among other things.
The Dresden Files loves this trope. Legendary creatures from the folklore of every part of the world exist, deities from all pantheons are real (though some aren't active anymore), functional magic is an everyday reality, etc. It is heavily implied that these things are not real because they are myths, but myths because they are real - people who had experiences with non-mortals told stories about them that the mortal world consumed. For example, Bram Stoker's Dracula was commissioned by the White Council to educate people about Black Court vampires, who were prevalent at the time (and it worked - Black Court vampires are very rare in the series). Also, Mab has a autographed, personalized original copy of Grimms' Fairy Tales. One reason this might occur in the Dresdenverse is that most Gods Need Prayer Badly.
In the Shadowspawn series, the title beings are the basic for just about every legend of magic or monsters (especially vampires and werecritters) there is.
In Anthony C. Gilbert's Farther Up and Farther InAll Myths Are True about life after death. Except, apparently, the belief that there isn't any, because the narrator is an atheist but gets sent to Hell, the Christian afterlife being the default for Westerners without other positive beliefs. Escaping from Hell (!) leads to a Crossover Cosmology where he meets Freja, Pan, Monkey and others: the final message (logically, given the opening premise) is that All Gods Are One and we are One with them.
In Rainbow Mars by Larry Niven, all of the Martian legends are true, from H. 'G. Wells to Edgar Rice Burroughs.
Robert E. Howard's Conan the Barbarian has a number of gods that would, in Howard's world become the basis of more modern deities. Crom, Lir, Babd, Macha and Nemain are all Celtic, the Hyborian Mitra becomes Mithra, who's also something of a Crystal Dragon Jesus, the Shemite Ishtar becomes the Babylonian Ishtar, the Turanian/Hyrkanian Erlik becomes the Mongolian Erlik and the Stygian Set seems to be the basis for both the Egyptian Set and Apep
In the Tales of the Five Hundred Kingdoms by Mercedes Lackey, all fairy tales, from Russian to Middle Eastern to the Brothers Grimm and anything else, are true. In fact, a magical force known as the Tradition actively works to try and make them come true. Those that are Genre Savvy will use the Tradition to their advantage.
The Bifrost Guardians by Mickey Zucker Reichert is another all myths are true, with the melding of technology to Norse myths to Christianity.
This is the premise of the novel Out of Their Minds by Clifford Simak.
In Prince Caspian from The Chronicles of Narnia: Caspian was always taught that the "old Narnians" were myths and fairy tales, then he learns that they are in fact real. The appearance of the Pevensies and Aslan also turns out to be this for many old Narnians.
In Orson Scott Card's The Lost Gate the Westillian Families are the basis of all Indo-European pantheons. It is inferred that other cultures deities, including the Abrahamic one have similar origins.
The universe is itself shaped by belief—if enough people believe something is true, it becomes true. Hogfather explores the extremes of this idea with such characters as the Verruca Gnome and Bilius, the Oh God of Hangovers.
In Djelibeybi, as seen in Pyramids, the Fantasy Counterpart Culture of Egypt, all myths are true concerning their gods, which have been evolving and developing for seven thousand years. The priests are said to "never throw away a god in case they turn out to be useful" and to be able to "give headroom to a collection of ideas that would have made a theoretical physicist give in and hand in his badge" (paraphrased).
In the Dune novels, the Bene Gesserit have a whole system of false myths called the Missionaria Protectiva. They purposely spread made-up prophecies that any member of their order can fulfill if needed. Thus, a member stranded on an otherwise hostile world can appear to be The Woman From the Prophecy.
In Brandon Sanderson's The Well of Ascension, the Twist Ending is that the prophecies have been deliberately altered by a powerful being in order to manipulate humanity/the heroes into freeing it.
In His Dark Materials, many divining methods are actually just "talking to Dust", the sentient matter forming most of the universe.
Young Wizards plays with this trope, in that many myths were inspired by the non-mythical actions of the godlike Powers That Be. For example, the extremely powerful Winged Defender is the inspiration for (among other things) Thor, Athena, Prometheus and the archangel Michael.
Lizzie visits several worlds in Spider Circus and encounters werewolves, vampires, selkies and more. It seems that people who can travel to other worlds are responsible for the spreading of these myths.
Zeus Is Dead: A Monstrously Inconvenient Adventure, despite its premise of the Greek gods making a public return to the modern world, also averts this: Hera (at the gods' first press conference) specifically states that while the Olympian gods ARE real, other gods such as Thor, Loki, Anubis, and Elvis Presley are not.
The Spiral Series doesn't restrict itself to historical myths. Everything that has, can or even will be imagined can be present at some point within the Spiral.
Subverted as a Running Gag throughout Buffy the Vampire Slayer and Angel. Despite living in a world where vampires, werewolves, witches, dragons, demons, and zombies are all real and have been encountered by the main cast at one point or another, everyone agrees completely unanimously that leprechauns aren't real. Unfortunately, Santa is also real, a child-eating demon who comes down chimneys.
Fringe started off seeming like a Spiritual Successor to The X-Files, focusing on a unit of the FBI investigating seemingly paranormal phenomena and switching between Monster of the Week episodes and Myth Arc episodes. However, it's gradually revealed that everything paranormal is part of one pattern that began when Walter crossed into an alternate universe to save that universe's version of his son. In other words, what seems paranormal is only pseudoscientific. Also, in one episode, Walter makes it clear that while he believes in many things, he draws the line at ghosts.
The final Quatermass serial has stone circles (which do nothing; the stones only mark the places where people congregated in the past) around the world becoming activated; people congregate there (an activated race memory), expecting to be: contacted by aliens, 'raptured' into heaven, 'go to the planet', etc. Instead, they are 'harvested' by an interstellar energy beam that reduces them to dust, with a tiny fraction lost to the beam. It is further suggested that all religions, and by extension, all of human politics, wars and history, have been the result of this race memory: to congregate and be harvested.
Averted in Babylon 5's episode "A Late Delivery From Avalon"—an arrival on the Babylon 5 station claims to be King Arthur, brought back among humanity after a long hiatus (when he was "taken to Avalon" on a mysterious "ship"). There is actual discussion among the main characters as to whether this could be true, since there was already a known case where the Vorlons did abduct a historical figure and used him to do their bidding in other times ( "Comes the Inquisitor"). But it turns out he was from the present time, suffering trauma-induced delusion from being the officer who fired the first shot that started the Earth-Minbari War years ago.
Angels? Check. Vampires? Check. Titans? Check. Human representations of the animals in the Chinese calendar? Sure, why the hell not?
An interesting case occurs with Pandora's Box. In the season 3 episode "Sin Francisco", Leo claims a box holding the seven deadly sins inspired the legend of Pandora's Box, implying it didn't exist. The season 7 episode "Little Box of Horrors" would have the Charmed Ones deal with the actual Pandora's Box.
In Lost Girl, all mythological creatures (including phoenixes and unicorns) are real and are really Fae. Kenzi takes it all in stride, then becomes absolutely terrified when she realizes this means Baba Yaga is real. Later, nobody can find any references to the Garuda, because the race actually predates the Fae and no myths or legends of them were written or told.
Let's not forget Supernatural, were literally everything but Bigfoot, and possibly unicorns, has been confirmed to exist in some shape or form. Makes you feel sorry for the poor bastards who exist in that reality, given how practically every monster, folklore, wives tale, myth and legend either hates humanity or loves to eat humanity.
Subverted in Mage: The Awakening, where part of being a mage is sorting through which myths are true and which are not. Note that, in this case, "true" probably means "contains a tiny kernel of actual supernatural, historical or cosmic insight which was either implanted or leaked through into the human consciousness", while "not true" probably means "was deliberately fabricated by other mages in order to mislead those who would seek the truth, was deliberately fabricated by other mages in order manipulate the course of human culture, or was just a myth that people came up with".
There is a general World of Darkness skill called "Occult" which allows you to attempt to tell whether some myth is true or not. This is exceptionally valuable to Mages because of the aforementioned nature of their power: if they get it wrong, it may be correct for exactly as long as they're paying attention to it, then revert to the way it actually really is the moment they stop paying attention. This can be even worse than getting it wrong in the first place. Consider trapping a vampire in a corner by leaving a holy symbol in the middle of the room, then going out for coffee... with the vampire no longer trapped the moment your back is turned.
The Eberron setting of Dungeons & Dragons has a version of this: in contrast to most settings in which Clerics derive their powers from one of a set of specific gods chosen by the DM and which definitely and demonstrably exist, in Eberron, anything a cleric believes in sufficiently works. Clerics can be devoted to abstract concepts (like "Justice") instead of specific gods, and there's at least one religion in which the followers know their god doesn't exist (yet) because they're in the process of building him.
In RuneQuest, the different races and cultures each have their own religion, with mutually incompatible stories about the creation of the world and the nature of reality. However, in-game, all myths are true, and priests of the rival pantheons are equally powerful.
Shadowrun has everything from Sasquatch to both Western and Eastern Dragons reappearing after 2012. And pretty much every tradition of magic actually works now and summons different spirits ranging from angels to kami to nature spirits.
Played with humorously in Oblivion where Sheogorath asks you (or you ask yourself if you've become Sheogorath) to fulfill a prophecy a small village has about the end of the world that includes attacks by rats and FLAMING DOGS DROPPING FROM THE SKY. The prophecy is used as little more than a prank.
The Game Boy game Final Fantasy Legend II (SaGa 2 in Japan) avoids this. One world your characters explore has a myth that turns out to be true and another myth that turns out to be false. Also, there are actually 78 "MAGI", not just 77 as mentioned at the beginning of the game.
Professor Frankly encounters conflicting theories on the nature of the treasure he's looking for. Eventually one of the theories turns out to be true: the treasure is an ancient demon. But later it is revealed that the 'real' treasure was a Dried Shroom, the weakest healing item in the game.
In the same district of Rogueport that Frankly's house is located in, you can find a quirky storyteller who is glad to spin all sorts of old stories. But that tale about the horrible evil monster and the four heroes who fought it before being themselves sealed away couldn't be true, right? Of course it is. The monster is a demon sleeping underneath Rogueport right now and Mario actually encounters each of the heroes in the form of talking cursed treasure chests. They're pretty nice.
In Fire Emblem: Path of Radiance, the heroes learn that a medallion holds a dark god who will bring The End of the World as We Know It if freed, and it can be freed by Magic Music or a huge war. The fact that certain people can become mindless berserkers by wielding the relic reinforces this belief. But in the sequel, Radiant Dawn it turns out to be a lie spread by the Dragon Laguz king in vain hopes that it'll prevent war between everyone in Tellius. In truth, endless war will actually awaken the goddess Ashera, who will see the wars as a sign that those living in Tellius are failures, and must be purged away to allow for a perfect world.
In the Shadow Hearts series all myths are true, though very often in ridiculous, bizarre and over the top ways.
Some of the local legends recounted to the protagonists of Chrono Cross turn out to be... slightly skewed.
Averted in Star Control II. The Black Spathi Squadron is fictional and has no effect on the plot, and many religious beliefs and local legends are brought up which are never mentioned or investigated again.
In the Dark Parables series of PC games, all fairy tales are true - and interconnected. Their sister series Cursery says that all nursery rhymes are true and created by Mother Goose herself.
Metro Last Light does this to a fair degree. While the novel of Metro 2033 was pretty steeped in ambiguity, Metro Last Light takes a different route with it's storytelling. The nosalis rhino is said to be a legend one level before it's fought and, of course, who could forget: "You may still harbor dreams of looking for some legendary artifacts, like the proverbial Map of Secret Metro, but... I think I'll have to disappoint you."
In Mortal Kombat 9's story mode, Nightwolf can sometimes be heard referring to Raiden as "Haokah", the lightning spirit of the Lakota tribe, giving credence that Raiden has at least appeared to their culture and is likely the god of thunder to anyone else who had one (Zeus, Heracles, Thor, etc.).
Of all Shin Megami Tensei line ups so far, Persona series might be the most fitting in this trope because it's the most mundane and closest to current Earth in term of setting and mythology. People aware of the legends and often discuss it if the topic is brought up, but only select few are aware that those gods are exist and can be summoned. Since this particular verse, especially in Persona 2, is run by Clap Your Hands If You Believe rule, if rumor circulates and people en masse come to believe it, it will without fail becomes reality. And since mythology exists in the first place as a belief, you do the math. The exact mechanic of it are varies depending on the particular works, but the basic is the same.
Persona 3: A local apocalypse cult that worship a Goddess of Death gains power and influence among public, which then empowers said Goddess of Death into granting them said apocalypse. Also, your school nurse's rambling about Tarot Arcana is true through Social Link.
Persona 4: The Midnight Channel, a television channel that only appears in a rainy midnight hour will foresight your future soulmate. The second part isn't exactly true; the channel only shows a person the audience wants to see. Within the channel, a Shadow of that person will take form of what the audience wants to see combined with what said person actually is. If that person is rumored to be a man when she's actually insecure and hiding her gender, the Shadow has its appearance twisted and takes form of a genderbender.
In the Castlevania series just about every fictional creature has appeared at some point, mostly as the enemies the player fights.
BlazBlue borrows seiðr (here rendered as "seither"), as well as symbolism of Ragnarök and Ratatoskr from Norse Myth. It also has playable vampires and werewolves. Though the heaviest influence is Shinto, with Murakumo (Nu), Susanooh (Hakumen) and Orochi (Black Beast) being integral to the Ground Hog Day Loop of Calamity Trigger. And in Chronophantasma, the Bigger Bad is revealed to be none other than Izanami herself.
The Nasuverse is filled with nearly every mythological being you could possibly think of, plus a few more. Vampires, werewolves, two different types of ghosts, demons, unicorns, so many more monsters, gods and of course, the incredibly complicated Functional Magic. Fate/stay night has the Holy Grail War, with the Masters summoning Servants that are based on heroes that actually existed there except for Fake Assassin.
Myths and superstitions in Tales of the Questor tend to be problematic after a few too many generations. Some of them end up being accurate, but for each one that actually is, you've got a few dozen that are corrupted from translation issues or pure age, and hundreds that are plain false or started up from illogical premises. It's also a rule for the setting that no one can see the future, so prophecy tends to always be wrong.
Subverted in thisAbe Kroenen comic. Of course, everyone present takes the fact that Atlantis exists in the first place as unsurprising.
Wayward Sons focuses mainly on Greek mythology, but features figures from several other ancient cultures.
This short story from Skin Deep. "How am I supposed to know what is actually fiction around here anymore?"
In Hexenringe, all legends and myths known in the real world are based on actual events in Xanadan (faerie dimension) and when any legends and myths (or any story, actually) are recorded or represented through creative means, such versions appear in Märagan (Imaginary dimension).
Holy Bibble starts from the origin of the universe and begins to unfold the story of all myths, using character types (such as the God of War) to merge multiple gods into one person.
The main driving plot of Sire. All works of classic literature actually happened and the protagonists passed on their narratives down the bloodlines, bestowing their descendants with gifts and curses relating to their stories.
Tasakeru: True in-universe. Each sentient sees their species' version of the God of Time the first time Zero becomes his keshin.
In Phaeton if a myth isn't true already then it can become true through the power of the Enigma. Sometimes mythical versions of creatures who already exist become true, that's when things get complicated.
The Wanderers Library acts as a multi-universal, and as such, almost anything can be found within, from the grim reaper to Native American gods.
In Tales of the Big Bad Wolf there is the belief that myths are true accounts of past, present, and future of some characters but this point is arguable. It appears that "some" may be under debate as that has some problematic consequences for the figures who have a counterpart in fairy tales and myths.
Parodied in this collection of ironic Teach the Controversy t-Shirts.
In Ben 10: Alien Force, Grandpa Max reveals to Ben and co. that many beasts and cryptids are real and in fact aliens... but he dismisses Bigfoot as just some guy in a costume.
In one episode of The Real Ghostbusters, the Ghostbusters must deal with a creature from Irish folklore. According to legend, the creature can only be stopped by a four-leaf clover. All the characters go out searching for one, except Egon, who, playing the role of Agent Scully, insists that the creature can be captured using the same "scientific" methods they always use. In the end, the four-leaf clover fails (it was a fake taken from a parade float), and Egon saves the day by capturing the creature "scientifically", exactly as he said he would. Despite this exception, the show generally followed this trope faithfully, as did its Sequel SeriesExtreme Ghostbusters.
Gargoyles took the stance that "All things are true, few things are accurate."
In the pilot episode of My Little Pony: Friendship Is Magic, the legend of Nightmare Moon and the Elements of Harmony are dismissed by all as an "old pony's tale", even by the millennia-old Princess Celestia. Of course, not only does it turn out that they're both real, but Nightmare Moon is the younger sister of Celestia, who deliberately passed them off as myths so her student Twilight Sparkle could fulfill a prophecy to return her sister to normal.
BIONICLE has made liberal use of this, though most of the myths have been distorted through the ages, and the rest have other things keeping them from being perfectly straight examples:
In the first few years of the franchise, each time a new threat appeared, the Turaga elders had a legend ready to explain their presence. Eventually, the Toa got rather annoyed with being kept out of the loop until the last minute, finally getting the Turaga to explain just where they got all their information:
The original backstory said that the Great Spirit brought the Matoran out of darkness to the island of Mata Nui. We later find out that it was actually the Turaga who rescued them (as Toa Metru) from their ruined city of Metru Nui, they just credited the Spirit with giving them the strength and abilities to do so. (They also treated Metru Nui's existence as a Greatest Story Never Told to keep the Matoran from remembering and getting homesick.)
One story said that poor workers were sent to the dreaded realm of Karzahni to be punished. In truth, poor workers were sent to Karzahni to be fixed; it's just that Karzahni was a really crappy healer and he never let anyone leave.
One legend that isn't real is that of the monster Irnakk — that is, it wasn't real, until the Piraka entered an area that brought worst fears to life... (Thankfully, Irnakk only existed briefly before vanishing.)
Steampunk-themed band Abney Park explores this in many of their songs, most notably in the aptly named "All the Myths are True".