A Kind of One

Some mythological creatures are more famous than others, but a few are so famous that their entire species has been named after them in the popular consciousness, in a kind of mythological malapropism -- or, alternatively, a singular creature proposed as a unique aberration is adopted by other stories as if it were a species. The ancient phrase for this trope is sui generis, roughly "a thing that defines its own category".

Related but distinct from Single Specimen Species, since the original creature may have less-famous forgotten relatives, or the original creature might have been solitary and then turned into a race by later authors. Either way, there's not just one anymore.

However, A Kind of One may be portrayed as a Single Specimen Species in each subsequent work, following the original; in which case it may be a case of Call a Smeerp a Rabbit since the characters have never seen it before, but the readers have.

For instance, someone encountering a creature for the first time may Call a Pegasus a Hippogriff.

Trope name is a reversal of "one of a kind".

See also Trope Namer, Ur Example, Public Domain Character.

Mythology

 * Chimera - A unique creature, child of Echidna and Typhoeos (aka Typhon) in one version of the Greek myth. It's now synonymous with Mix-and-Match Critters (and has a similar meaning in Real Life genetics). It has also been used to describe an illusory threat.
 * (The) Hydra - Again, a singular creature so tough that it took Heracles a labor to beat, yet is often a random encounter in many a Role Playing Game.
 * Medusa - She was only one of three Gorgons, her sisters being Stheno and Euryale.
 * The God of War series alternately refers to the monsters as Medusas or Gorgons, but Euryale is fought in the second game.
 * Averted in American Dragon Jake Long, where Jake fights the Gorgon Sisters: Fury, Euryale and Medusa; with Fury being the leader and receiving the most screentime. This however makes a different mistake by conflating/confusing the Gorgons with the Furies.
 * Stheno was a random encounter in (of all things) Final Fantasy Mystic Quest, the super-short kiddie-lite version of Final Fantasy.
 * Magic: The Gathering totally has gorgons, but in small amounts, instead of Medusa. This is Older Than They Think.
 * Stheno is also a very early Boss (a leader of naga-like snake people) encountered at the low levels of City of Villains
 * More specifically, her minions, the Snakes, are encountered at very low levels. You don't actually encounter Stheno herself until you hit the highest level tier and travel back to the starting island to beat her up.
 * Nethack also gets it right in that "Medusa" is a boss and there is only one of her. Better yet, her lair includes a statue of Perseus...
 * Ditto several Castlevania games, which almost makes up for having "Medusa heads" as mooks in almost every installment.
 * Nothing makes up for that.
 * Averted in The Kindly Ones arc of The Sandman, where Lyta meets the two remaining sisters who are still in mourning for Medusa.
 * They also show up (or images of them) in Fate/Hollow Ataraxia, though like Medusa before they assumed their monstrous forms.
 * Medusas are a monster race in Dungeons and Dragons, apparently female-only; the 2nd edition Monstrous Manual included a male-only version with the power to restore petrified things to normal, suggesting they were a single species dovetailing their abilities. Since this was stupid and reduced the terrifying impact of the medusa, they are almost uniformly forgotten or ignored. 4th edition adds male medusas back. They don't have snake hair and have a venomous gaze instead of a petrifying one. Gorgons, meanwhile, are metal-headed bulls that breathe petrifying gas.
 * Pardus has a jellyfish-like species named Medusa for its petrifying abilities... and its stronger relatives, Stheno and Eurylae.
 * Pegasus - There was only the one in myth, but has since become synonymous with pterippi, or winged horses. Interestingly, Pegasus was more or less Medusa's son. He... sort of spawned from the blood that spilled when Perseus beheaded her.
 * In Dungeons and Dragons (older versions at least), each Medusa killed has a chance of spawning a Pegasus in this way.
 * Minotaur - Another unique creature naming an entire species. The original was the result of an affair between Minos' wife in a sex-bot shaped like a cow and a magical bull from out of the ocean. Granted, this one does make some sense at least: Minotaur means bull of Minos, and in some versions the original Minotaur's proper name was Asterion, which would kind of make "Minotaur" even more of a descriptive, species-like name in the first place.
 * Inverted in Oedipus the King: Sophocles, who was familiar with the many unique monsters in Greek mythology, treats "The Sphinx" as a unique monster. But in Egyptian mythology, sphinxes are a species with many members.
 * Depending on the story, the Phoenix and the Unicorn are often portrayed as being mortal, but only one existing at a time. The Phoenix, for example, is reborn from its own ashes.
 * Lamia was a queen of Libya who became a child eating demon in Greek mythology. Over time she came to be generalized into a broad category of succubi, vampires, and other monsters called lamiae. It also became a name for witches and harlots.
 * Empusa was originally a daughter of Hecate with flaming hair who seduced men before drinking their blood and eating them. She was eventually demoted in mythology to a class of spirits called empuse, who served Hecate by guarding roads against unwanted travelers. Later, they were further demoted to a kind of hobgoblin that bothers Greek farmers in the form of various animals.
 * Empusa's other parent, Mormo, was similarly a single spirit who bit bad children that was generalized in to the Mormolyceion.

Literature

 * Frankenstein - There are dozens of different kinds of Golems and reanimated humans, but Frankenstein's Monster has become a catch-all. And don't forget the fallacy of calling the monster Frankenstein. (In the book, he has no name at all, though the author sometimes referred to him as "Adam".)
 * Technically, since the real "Frankenstein," Konrad Dippel (he was from the domain of the Von Frankensteins) was a dabbler in Hermeticism as well as science, the monster is probably properly classed as a Homunculus.

Film

 * Yoda from Star Wars probably deserves a mention here, since his species name was never revealed, and any other member that shows up is invariably referred to as "a yoda".
 * George Lucas himself is said not to want the species named, and so it's almost universally referred to as 'Yoda's Species'. At one point he was even reluctant to allow any other characters of Yoda's Species to be introduced. Apparently he hasn't shared his reasoning for why Yoda's background should be so mysterious.

Video Games

 * Yoshi is a species of dinosaur most often referred to as "Yoshis" in all games with others of his kind, with the exception of Super Mario RPG, in which they were "Yo'sters."
 * Also true of Toads (though they used to be called Little Toadies) and Birdos.

Web Animation

 * The Cheat from Homestar Runner looks vaguely like a short, fat miniature cheetah; he's been referred to as "a The Cheat" and no other creature resembling him has ever been seen (except in Cheat Commandos, which is purely metafictional, so all the Cheat Commandos characters may be depictions of him). One cartoon shows him hatching out of an enormous egg along with "a lifetime supply of fishsticks" and one of the video games has Strong Bad referring to his "hot mom", though, so possibly there are others.

Western Animation

 * A rather odd example in Futurama. Observing his behavior, Leela names a pet oh-so-cute creature "Nibbler". What does his species ultimately turn out to be called? "Nibblonians". Possible explanations include that they all do like eating things, and that the species name is only heard by psychic translation, so it may just have been their choice for her.
 * Dinosaur: "Look at all the Aladars!"

Tabletop Games

 * Honorable mention to Uncle Istvan from Magic: The Gathering, whose creature type used to be "Uncle Istvan" (later changed to "human").
 * But you can still have several Uncle Istvans in your deck, since he isn't legendary.
 * It's older than that: Several creatures from Arabian Nights had unique creature types: Ali Biba, Ali from Cairo, Aladdin. If that set were made today, they'd be legendary.

Real Life

 * Caesar - Well, there were many Caesars because it was Julius' family name. It then became a term for emperors, spawning both Czar and Kaiser.
 * Note that Caesar is derived from caesaries, meaning (head/long) hair.
 * There are a couple different explanations: it could be cum caesarie (with hair), with oculis caesis (grey eyes), or, indeed, caeso matris utero (born by tearing the mother's womb). Julius Caesar himself was certainly not born by Caesarean section; it was inevitably fatal at that time, and his mother survived his birth by many years. It would have been the ancestor who originally brought the name into the family.
 * Renard, in France. Before, the French word for fox was "goupil", but after the Roman de Renard, the whole species got the name. Renard (or Reynaud, i.e. Ronald) was a famous Trickster Archetype who was Cunning Like a Fox.
 * The Sun, the Moon and the Galaxy... are the generally-accepted proper names of Earth's sun, moon and galaxy, after they turned out not to be unique.
 * The Galaxy is usually called the Milky Way.
 * Two partial examples are the words "ocean" and "psalm," which, while not categories of one, are categories with specific, defined members. An ocean is one of four and only four specific bodies of water: the Atlantic, the Pacific, the Indian, and the Arctic. A psalm is one of the 150 songs or poems in the Book of Psalms. There presumably are not and can never be any other oceans or psalms.
 * However, there is some confusion as to how the oceans should be divided.