Single-Specimen Species

""But the most wonderful thing about tiggers is... I'm the only one!""

- Tigger from Winnie the Pooh

Something that applies to many a videogame boss (especially in the case of a Giant Space Flea From Nowhere), Kaiju or Monster of the Week - you come across a creature that looks unlike anything else you've seen or will see in that work's universe, but only once.

Bonus Points if it would be particularly hard for an ecosystem to support more than one of it.

Creators contemplating for the existence of more of these tends to be a gap in their World Building, but tends to be a case where the MST3K Mantra applies in full force.

However, for Long Runners, this may become a temporary trope, as a family/species and natural habitat may be created for the being in question at a later time.

Obviously different from the One-Gender Race as there's only one specimen, and the concept of gender may not even apply. Also different from Last of His Kind, as that implies the existence of more of the same species in the past, or A Kind of One, where a whole species is known by the name of its most famous member, who may initially have been this.

Note that a Single-Specimen Species is defined as being basically unexplained and therefore ecologically implausible. So the Last of His Kind can't "become" this, since that includes human survivors of a lost tribe, animals that were wiped out and so forth. However, A Kind of One may become a Single-Specimen Species in subsequent works, following the original; in which case it may be a case of Call a Smeerp a Rabbit since the characters have certainly never seen it before, but the readers have.

Not to be confused with Only You Can Repopulate My Race (which is ecologically implausible for entirely different reasons).

Anime and Manga

 * The Buyon from Dragon Ball, which could resist a Kamehameha in a relatively early stage of the series, was present only in a "boss tower" surrounded by an ecosystem in which it couldn't survive (Fridge Logic can be an important part of this trope — in this case, how did they bring the creature into the tower, and where from?). No other specimen ever appears again for the whole series, but it's interesting to see what would have happened if invaders had come across others of the same species — if others existed at all.
 * In Dragonball Z, Cell would apply since he's an artificially created being, but only until he makes some offspring of his own. Something similar happens with Majin Buu (also created, but by magic) in the same series until some variations pop up.
 * All the Angels in Neon Genesis Evangelion were basically this.
 * All the monstrous Angels actually had a common parent, the equally Eldritch Adam. Its ability to breed with Lilith (the other progenitor, this time of humanity) indicates that both are of the same species. By extension, this means that humans are of the same species as the Angels attacking Earth. All this of course assumes that 'species' has the same parameters for Angels as it does for earthly creatures.
 * Both used and subverted in Toriko. Many animals are so bizarre you don't expect to see another specimen, but sometimes one will show up.
 * In the Pokémon anime many "legendary Pokémon" would count as one of these.
 * Mewtwo (Since he's a clone of Mew) and the Poké-Gods (IE: Dialga, Palkia, Giratia, and Arceus) are the only truly "one-of-a-kind" Pokémon. Lugia is shown to have a breeding population, and many of the Legendary Birds (Articuno, Zapdos, and Moltres) and Legendary Beasts (Raikou, Entei, and Suicune) are shown to have different members of the species encountered by the main heroes during the course of the series. Likewise, there has been shown to be more than one Deoxys in the series, and the Mew in Pokémon the First Movie is a different Mew from the one in Pokémon: Lucario and The Mystery of Mew.
 * Also, Latios and Latias are shown to be the offspring of another pair of Latios/Latias in Pokémon Heroes, and Shaymin in Pokémon: Giratina and The Sky Warrior is actually part of an entire group of Shaymin(s).
 * In the manga however, it keeps the tradition of only-one species with all Legendaries, with the exception of Deoxys

Film

 * Godzilla and other more traditional kaiju sometimes are shown to have offspring, so they're right out. However, we see no indication of a Gojiru population from which it might have sprang. King Kong might qualify, as to our knowledge we never see anything that could possibly breed with it in that island.
 * The new Peter Jackson movie about King Kong kicks this trope out the door, since it shows a lot of bones from Kong's species - he's made out to be the Last of His Kind.
 * The known Godzilla population is this-Godzilla (1954), Godzilla (Showa), Minya, Godzilla (Heisei), Godzilla Junior, Godzilla Millennium, Godzilla SOS, and Final Wars Godzilla. However, thanks to Canon Discontinuity, it's hard to tell if there are any others or of what relation they all are to one another in the first place.
 * The Sarlacc and the Space Worm, prior to the somewhat dubious explanation for each in the Extended Universe novels and books.
 * Subverted in the film Rodan. The eponymous mutated Pteranodons are shown to be a breeding pair.
 * Averted with the Gyaos in the Heisei Gamera films. They can reproduce asexually and are soon able to attack in large swarms.
 * Likewise, subverted with Gamera himself. At first glace, it seems that he's the only one of his species. Then, during Revenge Of Irys, we see that there were thousands of Gamera that existed before him, but were all killed out leaving him to be the last of his kind.
 * Gonzo from The Muppets was this for a while, since he himself didn't even know what he was. Invariable he was referred to as something like "a whatever he is".

Literature
""The Halflings, cap'n, they will na take the strain" "Strider, we've got to get out of this snow. Legolas, did you get a reading on that creature?" "Fascinating, Captain. It appears to be an unknown creature that lurks in the pool waiting for passing strangers. Ecologically Implausible, captain." "Do you know what it is?" ... "I believe I said it was unknown, Dr Gimli. Logically, if I knew what it was, then it wouldn't be unknown." "Cap'n, we're in some sort of temporal warp, stretching and deforming the plot. The snow should take place a day before our encounter with this beastie." ... "Boromir, put on that red armour...""
 * The Watcher In the Water in The Lord of the Rings. (also, Tom Bombadil.)
 * It is suggested that the Watcher is one of the "nameless things" Gandalf mentions that live deep, deep below Moria. This one just happened to find its way to the surface somehow.
 * Parodied by The Straight Dope here:


 * In the Mithgar books, which started as Tolkien fanfic complete with pseudo-Moria and pseudo-Watcher, there is eventually an explanation both for the ecology and how such a huge monster got into the isolated lake - it's a kraken, they're part of a Bizarre Sexual Dimorphism with dragons, and a dragon flew by and dropped it into the lake to annoy the dwarves.
 * The title "character" in The Illuminatus Trilogy: Leviathan is a gigantic single-celled organism at the bottom of the ocean, a result of the first cell division that just got bigger and bigger while the rest of life on Earth developed into multicellular organisms.
 * In The Last Continent, the Wizards of Unseen University visit an island populated only with single-specimen species. This is because the island is home to the god of evolution, who personally engineers each animal (and tweaks it as it goes along), and is blissfully unaware of things like sexual reproduction.
 * Adam in The Bible qualified, before Eve entered the picture. Not to mention God himself, of course.
 * The Incredibly Deadly Viper in A Series of Unfortunate Events
 * Tigger from Winnie the Pooh.

Live-Action TV

 * A number of entities and creatures in the various Star Trek series.
 * On The Original Series
 * The creature that was Jack The Ripper. An entity that fed on fear and death, who claimed to be as old as time.
 * The Guardian of Forever. While the ruins around it seem to imply that it was artificially created, it claims to have always been. (Or at least to be older than Earth's sun.)
 * On The Next Generation
 * Ronin is a ghost-like lifeform apparently from Earth that bonds symbiotically with a human female, giving her pleasure in exchange for some kind of sustenance.
 * Data was thought to be this, until Lore was discovered.
 * On Voyager
 * The Telepathic Pitcher Plant, a giant space monster that would Mind Control the crews of passing starships to fly into its mouth so it could eat the ship whole.
 * The Nebula Creature (a massive organism that Voyager mistook for a nebula.)
 * Bevvox, the founder of The Think Tank, a huge organism which previously wandered the stars by itself for thousands of years. Unlike some of the others listed here, instead of bragging about how ancient it is, it's apparently sensitive about its age.
 * One episode of How I Met Your Mother had Marshall and Lily discover a small animal living in their apartment that had the characteristics of both a cockroach and a mouse: they dub it a cockamouse. No such creature is ever heard of again after they threw it out the apartment window (don't worry; it can fly).
 * The cockamouse turns up in a later episode, in a room in a rundown hotel that Ted stays in for plot-related reasons. He cheerfully reports that it's had babies.
 * Many, many monsters of the week on The X-Files and The Outer Limits.
 * Averted in Stargate SG-1. It looks to be a straight example when we first meet the Unas (his name certainly gives the impression). We don't meet another until two seasons later at which point their nature as a species is explained (and Jack lampshades the confusing name.

Mythology

 * The Phoenix—the western one—was the only member of its species. "As singular as the phoenix" was a common comparision.

Tabletop RPG

 * In early editions of Dungeons & Dragons, any creature with a Frequency of "Unique" was this trope, as there was only one of them. Examples included demon lords, arch devils, deities and the tarrasque.
 * The fluff indicated this for the Tarrasque in later editions, as well, with the crossover-relevant proviso that unique means per world, not for the entire multiverse.

Video Games

 * Many a fantasy setting boss creature would qualify.
 * There appears to be more legendary Pokémon that are single specimen than the anime does, but it's hard to tell whether most are the only one, period, or just so rare they haven't found a second. Those almost certain to be one-of-a-kind include one-off bioengineering experiments Mewtwo and Genesect; the godlike Arceus and its creations Uxie, Mesprit, Azelf, Dialga, Palkia, and Giratina (though Arceus is known to make seconds of the latter three); also godlike Groudon, Kyogre, Rayquaza; and results of a unique transformation Reshiram, Zekrom, and Kyurem (Reshiram and Zekrom two halves of a great dragon according to legend; Kyurem theorized to the the remains of the original). On the other side, Latias is stated not to be one-of-a-kind, Deoxys at least has the potential for more to exist, and Manaphy certainly isn't unique as their eggs have been found.
 * In Pikmin, the Puffstool and Smoky Progg are fought only once and do not appear in the sequel. Averted in Pikmin 2 since any boss can be rebattled, even if killed the first time.
 * The Progg, however is claimed to be a mutant baby Mamuta by the end credits.
 * The Nihilanth in Half Life are similar biologically to other Xen creatures (especially the controllers), but apparently the only one of his species.
 * Justified by Word of God: the Nihilanth's species was systematically hunted down and exterminated by the Combine until only this one remained. Until Gordon killed it, anyway.
 * Forgotten Beasts in Dwarf Fortress are forgotten remnants from when the gods created a given world. Each one is unique as a result of being procedurally generated, and can vary in terms of limb count, tails, flight, dangerous excretions, and sometimes being made of inorganic things like glass or mist.
 * This gets weird when there is no feasible way for them to survive until they reach you. Creatures made of Mist, Fire, Water, and Dirt are universally killed in one hit because according to the game mechanics they have no way of holding themselves together but it doesn't realize this until it starts calculating their wounds.
 * The Thorian in Mass Effect was the only known specimen of its species in existence. This is probably a good thing, as it was hostile to most other forms of life and liked to mind-control people through spores. All that's left of it now is its nearly planet-wide root system... hopefully it can't regrow itself from that.
 * It's implied that Tails from Sonic the Hedgehog is the only multiple-tailed fox on the planet.

Web Comics

 * In The Inexplicable Adventures of Bob, Molly qualified until Dean Martin cloned her to make Golly.

Western Animation

 * Just about any Kaiju in The Powerpuff Girls.
 * Most imaginary friends in Foster's Home for Imaginary Friends.
 * Each of the 627 genetic experiments in Lilo & Stitch: The Series and Leroy from Leroy And Stitch

Real Life

 * Tales of the Loch Ness Monster.
 * Unless it actually is a type of aquatic dinosaur, if it exists. That would make it a Last of His Kind scenario.
 * Scientists uncover Single-Specimen Species all the time. Frighteningly, it's often unknown if they are the last of a species wiped out by human activity or the tip of an iceberg that is never uncovered because a second specimen is never identified.
 * Some long-extinct species are known from only one fossil (hence a single specimen), which can make it difficult to know which features were common to the whole species and which were specific to the organism; for example, the specimen may have had a genetic disorder, or may be younger (and hence smaller) than average. Also, even after other fossils are uncovered, the original discovery may be so iconic that it is forever tied to the species, especially if the original has a name: far more people know about "Lucy" than about Australopithecus afarensis.
 * These examples overlap with Last of His Kind
 * Most noteably, Lonesome George, the very last Pinta Island Tortoise.
 * Hyophorbe amaricaulis, the Loneliest Palm. This is a species of palm tree with only one known member in the world, in a garden in Mauritius.
 * Several animal hybrids exist that have had only one known representative, such as Harjeeven the half-African/half-Asian elephant calf (deceased) born at Chester Zoo. Most such hybrids arise in captivity, as species that interbreed in the wild tend to do so more frequently than this trope demands.