Call a Pegasus a Hippogriff

The daughter trope of Our Monsters Are Different and Call a Rabbit a Smeerp.

So your characters are on an adventure in a Magical Land, and they naturally run into a mythical creature. Said creature is then identified in the text or dialogue by the name of a similar (or not) mythical being or fantasy creature. Cue a moment of confusion for the viewer.

Could be employed just to underline in red crayon that Your Monsters Are Different. Alternatively, of course, the writer Did Not Do the Research -- or did a little too much research, finding an extremely obscure name or form of a familiar creature. This is a common cheat when fishing for names for Palette Swap Underground Monkeys.

This isn't quite Sadly Mythtaken as the very fact that the writer knows that mythical creatures have specific names implies doing some research. (Sadly Mythtaken is more for The Theme Park Version / Disneyfication of classic myths.)

In case you're wondering, the most commonly accepted generic term for Winged Horses is "pterripi". However they're often simply called "pegasi/pegasus" after the most famous example -- see A Kind of One.

Compare Istanbul (Not Constantinople), which is similar but for place names.

When a completely fantastical character is named after a commonly-known creature, see Call a Smeerp a Rabbit, which is a sister trope. The title is a takeoff on Call a Rabbit a Smeerp, and is a reference to one of the best-known examples.

Anime & Manga

 * In Mahou Sensei Negima there's an in-story example when the group encounters a monstrous dog creature with multiple heads. Nodoka, being the high-fantasy book fan, identifies it as Orthrus by its snake-head tails. But at the same time, it has three heads total like Cerberus (whereas Orthrus had two), so she can't really identify it as anything. . Note that in some myths, Cerberus is depicted with a snake tail or with snakes on his back.
 * Later on, Yue and her classmates fight against a creature called a "Griffin Dragon". The only thing about it that was Dragon-like was a scaly tail and a pair of horns. Oh, and the Breath Weapon.
 * Cardcaptor Sakura has Cerberus. aka Keroberos, aka Kero-chan. His true form being a lion with wings, he looks nothing like a three-headed dog from Greek Mythology. He more resembles a griffin... or the Demon Prince Vapula.
 * The Winged Dragon of Ra looks more like a griffin or a rukh.
 * Gryphon from Bakugan is actually portrayed as a winged, three-headed monster with a lion's head, a goat's body, and a serpent's tail instead of a monster with a bird's head and wings and a lion's body.

Comic Books

 * Dream has three guardian beasts, one of which is a winged horse. This character is also identified as a hippogriff. Given that this is Neil Gaiman writing, it's likely an E. Nesbit tribute (see Literature below). Also, the dragon (four legs) is called a wyvern (two legs) - possibly Neil knew what he was talking about but none of the artists did.

Film

 * The Kraken from Clash of the Titans isn't the giant squid or crab monster of Scandinavian myth, but some kind of gigantic pseudo-Greek mermonster. In the original mythology, the sea monster that was going to destroy the city of Argos (unless they sacrificed Princess Andromeda) was called Cetus.
 * An early draft of the American Godzilla featured a rival monster called the Gryphon; however, it is described as an amalgam of mountain lion and bat rather than the traditional lion and eagle.
 * In Avatar the giant, reptilian, mountain dwelling, creatures are called "Banshees" by the humans. Granted, hearing the call of such a creature very well could signal the end of your life (the largest of which, called the Toruk, actually even means "the last shadow", as in the last one you'll ever see), but one would think that the first thing that came to mind when the humans saw them would be a dragon.
 * In Drag Me to Hell, the classic "man-goat" demon that is after the (female) protagonist is oddly called a lamia, a creature with vastly different representations in the folklore of different European countries but that is always said to be female and most often a beautiful seductress. This is adknowledged in one scene where the demon's shadow briefly resembles that of a young woman before morphing into its usual figure.

Literature
"It had a large head, with enormous open mouth and savage teeth; from its back arose great wings, armed with sharp hooks and prongs; it had stout legs in front, with projecting claws; but there were no legs behind,--the body running out into a long and powerful tail, finished off at the end with a barbed point. This tail was coiled up under him, the end sticking up just back of his wings."
 * In E. Nesbit's The Book Of Beasts, the hero must summon a creature identified as a hippogriff to save his city from a dragon. The creature that appears is what most people would identify as a pegasus, a winged horse. To be fair, you can't say that a hippogriff isn't a winged horse (or that a pegasus isn't technically part horse, part bird for that matter). It's also possible that Nesbit figured that the word pegasus must only refer to the Pegasus; this was ages before My Little Pony remember.
 * Actually Pliny the Elder mentioned Pegasi living in Aegypt. So the idea of numbers of Winged Horses existing is Older Than Print.
 * An older example is Frank Stockton's short story, The Griffin and the Minor Canon from 1885, in which the eponymous monster -- by its description -- is clearly a dragon. The story might actually be considered a Lampshade Hanging on this trope, as the dragon sees a statue of a griffin and assumes that (since it also has four legs, wings, etc.) he must be of the same species and that "griffin" is what humans call him. Got all that?
 * Actually, it's more accurately a wyvern or wyrm, as it has only two legs:


 * Sir Arthur Charles Fox-Davies warns against confusing the two in his A Complete Guide to Heraldry, so it was apparently a common Victorian mistake.
 * One of the stranger examples is in the book Thorn Ogres of Hagwood. A character wanders into the action about halfway through the story. He is a short humanoid with a big, big beard and he carries a lot of different tools and has a great talent for metalwork. He is identified as a dwa... no, wait, he is a Pooka. Pookas technically can appear as dwarves but, as you may recall from Harvey, they also tend to be a lot weirder.
 * Similarly, in Xanth, despite having clearly done the research (Anthony had the Naga being snakes with human heads rather than alt-named Lamia), the creatures he calls Pooka are... ghost horses bound to the living world with chains swathed around their bodies, who can't talk and have very shy temperaments. And can somehow reproduce (then again, it's Xanth.)
 * The original pooka was known for transforming into a large black horse that would give anyone foolish enough to mount it a terrifying ride, which is probably the origin for that bit of lore.
 * Similarly, mythical Veela are closer to Sirens, not the Succubi in the Harry Potter books.
 * Another Potter example is boggarts, which are not shapeshifters in English lore.
 * JRR Tolkien was fond of using "worm," the Middle and Modern English cognate of Old English "wyrm", to mean "dragon" or "serpent". "Worm" in the sense of "dragon" is attested as late as the mid-19th century in Northern English, as in the ballads of the Lambton Worm and the Laidly Worm, so the Good Professor wasn't just making it up as he went along.
 * It is even older - in the old North Germanic languages, "orm" could mean a snake, a worm or a dragon by modern English terms.
 * Although she never appears in the stories in person, it's made pretty clear that H.P. Lovecraft's Mother Hydra has nothing to do with the Hydra of the Greek myth.
 * In David Weber's Safehold books, the humans who have settled on the planet Safehold have named many local animals after mythical beasts. Examples include the kraken (described as a cross between a squid and a shark, fitting the latter's place in Safeholdian ecology), the dragon (a massive, six-legged animal that comes in both carnivorous and herbivorous varieties), and the wyvern (four-winged flyers that are the Safeholdian analogue of birds).
 * Arcana has "Unicorns," which resemble the usual image of unicorns only in that they have a single horn and are roughly horse-sized and shaped. They are black, with disproportionately long legs, powerful hindquarters, and ears like a bobcat -- and possess a mouthful of long tusks and sharp, carnivorous teeth.
 * There are carnivorous "Alicorns" (also called "One-Horns", but guess what unicorn means) in the Elvenbane series as well. Traditionally, this word refers to either winged unicorns or the horn of a unicorn, although it's likely a result of centuries of Recursive Translation from English <-> French (unicorn -> une icorne -> l'icorne -> a licorn -> alicorn).
 * Some of the main characters are shapeshifting superintelligent dragons who are, in some details, quite different.
 * In The Carpet People, there's an enigmatic, prescient race which most people would call "elves" based on the description. Instead they're "wights", which more commonly refers to minions of The Undead.

Live Action TV

 * Somewhat related: The creatures that attack Arthur and Merlin in the Merlin miniseries are Raptors with squirrel-like patagia no matter how much Merlin insists on calling them griffons.
 * No one has ever been able to work out why someone decided to call flying undead monsters dryads.
 * Rather than wild, intoxicated and lustful female followers of the Greek god Dionysus, maenads in Buffy the Vampire Slayer Expanded Universe are depicted as equally mad followers of an ancient vampire named Kakistos (whom Faith slayer later) who were prime cases of Torture Makes You Evil (or just plain crazy) and passed their tortures onto other unfortunate girls until their minds broke and served Kakistos as well.
 * Xena: Warrior Princess did the same, except it used the name "Bacchae", which actually referred to Bacchus' human female followers (at least, the maenads were nymphs).
 * In the second season of Mighty Morphin Power Rangers, the Zord that was previously described as a Pegasus (Tenma) as a unicorn, and the Qilin was described as a griffon.

Tabletop Games

 * Get a drink for this one. In Dungeons and Dragons, gorgons are a variation on the creature known as the catoblepas in more classical bestiaries. The creatures that resemble the Gorgons of Greek mythology are named medusas, after the best-known Gorgon. And as if that weren't confusing enough...the catoblepas, by that name, has actually appeared in some editions of D&D. (And while—unlike the previous two—it's always fit one version or another of the catoblepas myth, it's always been notably distinct from the gorgon.)
 * Ditto for Heroes of Might and Magic III.
 * Incidentally, the word "Gorgon" literally means "horror".
 * The use of the name "gorgon" for a bull-like creature comes from a particular medieval bestiary, which used that name for the catoblepas as a reference to the whole "kill with a glance" thing.
 * Lamia in Dungeons and Dragons have a confusing history due to Lamia's own historically confusing mythology as either a snake woman, a hermaphroditic hag, or as a four-legged beast with a woman's head & breasts. All editions on D&D up until 3rd have used the last one as inspiration for a lion-centaur monster which somewhat follows it. However, 4th edition just decides to chuck all mythology out the window and attach the name to a swarm of insects that crawl over the skeleton of a dead humanoid and use spells to disguise themselves as people.
 * At least one article in Dragon magazine has suggested that game masters use this trope in-game to screw with their players' expectations, perhaps justifying it as disinformation spread by Genre Savvy monsters.
 * Vampire: The Masquerade calls vampires' mortal servants "ghouls".
 * Magic: The Gathering's Innistrad block features "Griffins" that look an awful lot like hippogriffs... which is particularly confusing since "Hippogriff" is a separate creature type in the preceeding Scars of Mirrodin block. Mercadian Masques also features some decidedly odd trolls and satyrs. And then there're the Ravnican Nephilim which... don't resemble anything, much less Biblical giants.

Toys

 * Zoids doesn't even seem to care, considering robot triceratops with sharp pointy teeth are Rule of Cool, but to writ: the Liger series of zoids are referred to as 'lion type' and several four-legged-and-winged dragons are 'wyvern type.' For that matter, whether godzillaesque or more like a giant monitor lizard, any big reptile zoid made before 2000 will be called 'tyrannosaurus type.'

Video Games

 * Another bizarre example: World of Warcraft has large predatory cats with bat wings and scorpion stingers. They are called manti... no, they are called wyverns. (Wyverns are usually bipedal dragons; to be fair they also have tail-stingers but...) Apparently realizing the mistake, the "wyverns" are now almost exclusively referred to in the game as simply "wind riders."
 * World of Warcraft also has bipedal dragons. They are called proto-drakes and as the "proto" implies, they are weaker and at a lower Evolutionary Level than regular dragons: they are generally smaller, although there's a large range in dragon size, and are incapable of speech or using magic like most dragons.
 * Same thing with the wyverns, which look more like lion-bats http://www.wowwiki.com/Wyvern than two legged winged reptiles found in medieval heraldry as well as the chimeras, which looked like two headed wyverns http://www.wowwiki.com/Chimaera
 * Lammasu's look like an unsightly miniature cross between an Arokkoa (ugly bird-like humanoids in WOW) and a mole rather than the lion/bull bodied, human headed, winged Mesopotamian guardian deity depicted in Sumerian statues.
 * Hippogryphs in the Warcraft universe look like bird-stags instead of bird-horses and are often green, making them more resemble the mythological peryton.
 * In Castlevania Aria of Sorrow, the Cockatrice and the Basilisk are pretty much palette swaps, although they are completely separate critters. Most of the time.
 * The Gorgon/catoblepas issue (see Tabletop Games, above) is also present in Aria. Likewise, one of the most iconic Goddamn Bats of the entire series is the Medusa Head, which is very different from the other gorgons in the game.
 * The Kraken in City of Heroes is a giant blob monster that walks on two legs and resides in the scrappy zone of Perez Park and is a member of the villain group, the Hydra, which are all human-sized blobs. Except the Hydra, which is another humongous blob with tentacles that stretch all over the city. None of which ought to be confused with Lusca, the giant octopus which terrorizes Independence Port.
 * In a inversion of Dinosaurs Are Dragons, Monster Hunter refers to it's dinosaur-like monsters as Wyverns just like the actual ones. The sub-type "Bird Wyvern" is basically a Velociraptor ranging in size from a leopard to a cargo container, while "Brute Wyverns" are small Tyrannosaurs.
 * Fallout has Centaurs, mutated creatures which look like... Well, see for yourself.
 * Pokémon has two Dragon-type Pokemon called Altaria and Flygon that look absolutely un-dragonlike (Altaria resembles a giant bird, while Flygon looks very insectlike despite being part Ground-type). Also, Charizard, despite its draconic appearance, isn't a Dragon-type at all (it's actually a Fire/Flying-type).
 * These are justified, in that Altaria is meant to be a Peng (huge dragon-like bird) from Chinese mythology and Flygon is an adult antlion, which look like dragonflies and have the nickname "sand dragon", so Flygon's dragon typing is a pun. Charizard is not a dragon only because in the first generation it would be completely overpowered to have a dragon type starter. Gyarados is also not a dragon for the same reason: with their type combos, ice wouldn't be super effective on them like it is on the actual dragon-type Dragonite, and the only dragon type move of the generation was Dragon Rage, which did fixed damage.
 * Vibrava, which evolves into Flygon, is even more insectoid. Bagon and Shelgon aren't dragonlike either, though their final form Salamence is. Kingdra is based on a creature called a weedy sea dragon. Dialga isn't much like a dragon, though Palkia has more similarity to the European dragon, and Giratina's Origin Forme bears its resemblance to the Chinese dragon. Really, considering that most of the types don't refer to specific lifeforms (dragon being joined by bug and ghost in doing so), it's not surprising.
 * The Legend of Zelda seems to do this in-universe, with several very disparate creatures sharing the same name (in the original Japanese, at least). Apparently any small masked critter is a 'hiploop' and any large masked one is a 'zeeclock' whether insectoid, reptilian, or avian.
 * The Legend of Zelda Skyward Sword has Octoroks that more resemble Deku Scrubs.
 * According to one interpretation (which happens to be shared by series creator Shigeru Miyamoto) each Zelda title is a differently corrupted version of the same core story rather than an entirely new chapter in Hyrule's history, a notion which the unusual in-universe use of this trope would appear to support.
 * In Cornish folklore, a spriggan is a kind of goblin that can grow to giant size. In The Elder Scrolls, it's basically a dryad.
 * A few liberties were taken in the depiction of Greek myth's monsters in God of War, mainly for the sake of Rule of Cool.
 * Scylla looks like a leviathan than the 6-headed, dog-waisted cursed nymph from Homer's Odyssey and the Ovid, whereas Poseidon's Hippocampi half horse/half fish (or in this case, crustacean) mounts are often confused with leviathans.
 * The kraken looks suspiciously like the Kraken from the Clash of the Titans remake
 * Some of the monsters Kratos fights aren't even native to Greek mythology, such as the Irish Banshee and the Arabic Roc.
 * Although depictions of monsters from the Shin Megami Tensei series are more or less accurate, there are some monsters that are noticeably different from the original mold. Most egregious are most versions of Cerberus, which looks like a snake-tailed albino lion father than a 3-headed canine (although the 3-headed cerberus was used in Persona 3 as Team Pet Koromaru's persona).

Webcomics

 * Homestuck has liches that could be much more comparable to gargoyles (though the wings are not part of the monster design; they come from the prototyped crow).
 * While we're at it, the trolls are are a race of Cute Monster People whose infant forms are reminiscent of insects and whose life cycle and physiology is just a tad strange. While not a full example - myths about trolls rarely agree on anything - the Homestuck depiction was intended to be rooted more in the idea of internet trolls (that's all the characters were before Andrew Hussie decided to make them relevant to the plot), and as such are certainly divorced from the traditional brutish, man-eating monsters that live under bridges.
 * Brooke from Eerie Cuties and her people are called "melusines," even though traditionally, "Melusine" was a specific individual - what she was was a "nixie." It would be like calling gorgons "medusas." Oh, Wait!, that totally happens.

Western Animation

 * The Leviathan from Atlantis: The Lost Empire is actually still referred in-film as "a mythical sea serpent", but it is actually a giant mechanical lobster, as pointed out by the quotes "Jiminy Christmas, it's a machine!" and "I want that lobster served on a silver platter!"

Other

 * Sirens are often portrayed as being mermaid-like in appearance, even though they were originally closer to harpies in the myths.
 * Not to mention languages that conflate the two names.
 * The chimera is often portrayed as being similar in body structure to cerberus, with the goat head, lion head (which is often depicted as a male lion's head in modern media as opposed to the original female lion's head where the beast was generally considered in Greek mythology to be a female), and a dragon head all together in the front http://anwoanimalworld.wordpress.com/2011/03/17/new-chimera-dragontoy-miniature-has-3-heads/. In original Greek mythology the chimera had the body and front head of a lioness, a snake for a tail (which is still present in modern depictions), and a goat's head on its BACK at the center of the spine http://cs.fit.edu/~ryan/chimera.html
 * To confuse matters even more, the term "chimera" is often used as a generic term to refer to any Mix-and-Match Critters.