Faeries Don't Believe in Humans, Either

""So you don't believe in dragons? It is curiously true That the dragons you disparage Choose not to believe in you.""

- Jack Prelutsky, The Dragons Are Singing Tonight

Two races, or tribes, were familiar with each other's existence a long time ago but they have gone ages without contact with each other, and now each believes the other to be creatures of legend, that either never existed or died out long ago.

Until they meet each other. And they both react exactly the same way — by saying something along the lines of "They do exist!" in a tone of awe. At the same time. (Or at least one directly after the other.)

Compare Mutually Fictional, where the two sides really did originate as (or still are) only stories to each other.

Advertising
"Son: Dad, do aliens exist? Father: No, son, it's fiction! [Cut to an alien planet] Alien son: Dad, do humans exist? Alien father: No, son, it's fiction!"
 * This Christmas M&Ms ad. Verbatim with the trope even.
 * The Russian advertisements for Hochland cheese:

Film
"Fact is there's nothing out there you can't do, Yeah, even Santa Claus believes in you!"
 * Referenced and inverted in The Muppet Movie, in the Electric Mayhem's song "Can You Picture That":

Literature

 * In Through the Looking Glass, when Alice meets the Unicorn, it asks what she is. When told that she is a child, it replies, stunned, "I always thought they were fabulous monsters!" When Alice confesses that she always believed that unicorns were fabulous monsters, the Unicorn says, "Well, if you'll believe in me, I'll believe in you," to which Alice agrees.
 * In The Lion, The Witch, and The Wardrobe, Tumnus the faun reacts this way to Lucy the first time he meets her, and owns a book called Is Man a Myth?
 * In JRR Tolkien's Farmer Giles of Ham, younger dragons seem to think that the Knights are just a myth. The older ones know better, although they admit that they are few and far, and not a danger anymore.
 * Which is true. The King and his Knights are pretty useless. The only person who can effectively deal with the Dragon is a fat, red-headed farmer who doesn't like trespassers—even if they are scaly and breathe fire.
 * There's a picture book about a little monster who cannot sleep because he thinks there are human children under his bed. His parents desperately tries to tell him human children are just made-up creatures, but what works is when he meets a real kid and learn that while they are real, they are not dangerous.
 * The Dragon With the Girl Tattoo by Adam Roberts, a spoof of The Girl With the Dragon Tattoo, is set in a world where dragons are the dominant species, and humans are a myth.

Live Action TV

 * Before meeting each other, the Power Rangers (In Space) and Teenage Mutant Ninja Turtles ("Next Mutation" version) each thought the other was an urban legend.

Video Games

 * In the eroge Let's Meow Meow!, the main character gets an Unwanted Harem of animal-eared girls from another dimension. It is revealed that humans - called "monkey-people", because our ears look like those of monkeys to them - are believed to be mythical or at least extinct.

Web Comics

 * Referenced in CRFH when Roger meets two centaurs who think humans are mythical creatures. However, they're actually genetically engineered creatures who have been given this belief (and a steady diet of drugs) by the Evil Genius in question to avoid them questioning their surroundings.

Western Animation

 * In Fern Gully, humans have been away for so long that the fairies only remember them through legends and folk tales, resulting in most fairies not even believing in humans until of course a logging company arrives in the area.
 * Pops up at the end of the sleepover episode of Jimmy Neutron Boy Genius. The climax of the episode features a series of increasingly-nonsensical Catapult Nightmares involving the various characters and a flying vampiric pizza. The final nightmare turns out to belong to an actual pizza vampire, sleeping in his pizza box; when he explains his terrible dream to his wife, she consoles him that children don't exist, everyone knows that.
 * Played with in Ruby Gloom. The Luna Monsters believe that the Misery monster exists but only comes out once in a blue moon. The main cast (aside from Misery, at first) believed there was one Luna Monster. The main cast seems to be grossly misinformed about the Luna Monsters. And the Luna Monsters are probably also grossly misinformed about Misery.