In real life, hybrids between different species are often sterile, which precludes the possibility of their further hybridizing. But, especially in fiction, there are exceptions to this rule; often, fictional hybrids are capable of having children themselves. Two hybrids of the same kind can thus be able to have children that would technically be hybrids like themselves, and those children can in turn have children of their own. If there are enough hybrids of the same two species, they might be able to produce a completely new species combining the two original ones from whom the hybrids first arose.
This seems to be particularly common for half-elves, which are depicted fairly often as becoming self-perpetuating races independent of their original elven and human progenitors.
In Real Life this is called hybrid speciation.
- Monstress: Arcanics are the descendants of the immortal Ancients and their assorted human lovers over the past few centuries. Most consider them to have become a distinct species.
- Pony POV Series: It seems all hybrids are capable of reproducing and producing fertile offspring, and ponies seem to be able to breed with most anything.
- In the Dark World, the pegasi and griffons have effectively merged into a single race, the hippogriffs, to the point Rainbow Dash and Derpy are the only pure blooded pegasi left (Fluttercruel who now has full control of her and Fluttershy's body as Fluttershy is long dead is a Semi-Divine half-draconequus). Likewise, donkeys and earth ponies have merged into a single species, and unicorns and zebras have as well. It's implied by dialog that Discord may have had something to do with that.
- Codex Equestria:
- All changelings have an element of this, as despite Emperor Blackthorn's best efforts, Blackrose wants nothing to do with him and thus the two had to have their many offspring with other species. Thus most of the Changeling Queens and Kings are hybrids in some capacity, and so are their offspring in turn.
- The Sparkle Ponies are heavily implied to be hybrids between normal ponies and the mysterious Visitors, and have become a subrace of ponykind in and of themselves large enough that Queen Dazzleglow is able to get a primarily Sparkle Pony population of her rebuilt country.
- The War Gods: Half-elves aren't quite a stable sixth race, but they come close. If two half-elves have children, those kids will be half-elven even if the elven ancestors are several generations back. (A half-elf/elf pairing also produces half-elven children, but half-elf/human means the children are humans.)
- Chronicles of the Emerged World: When humans first entered the then elven-dominated Great Land many centuries in the series' past, the intermingling of the two races led to the creation of the race of the half-elves. While the elves eventually left the Great Land, the half-elves had by then become numerous enough to form their own nation in the Land of the Days and become the Great Land's dominant race... at least until their near-total genocide by Aster the Tyrant shortly before the start of the series.
- The Night Land: In an intentional horrifying example of this trope, numerous of the Night Land's monstrous inhabitants arose through the intermingling of humans with the entities that entered it many millennia in the book's past, as the Sun slowly went out and the Earth began to die. The Giants are the example most focused on in the book, described as a species of hideous, towering, warty humanoids "fathered of bestial humans and mothered of monsters".
- Oath Of Swords has the half-elf Purple Lords.
- Elcenia: Due to the nature of their genetics, Barashin halfbloods breed absolutely true. This is also why they're called "halfblood" instead of "half-elf".
"My father's human and my mother's elven, but I'd look the same if I had only one ancestor of either a hundred generations back, and some halfbloods do. It's the blood that's halved, not the actual descent."
- The Mortal Instruments: Played straight with the shadowhunters and the fairies, which are all the children of other shadowhunters and fairies (the first fairies were the common descendants of angels and demons). But it is inverted with the warlocks, who are always born between a demon and a human.
- Star Trek: Nearly all the major humanoid races can interbreed and produce viable offspring. There are hybrids of humans and vulcans, klingons, romulans, cardassians and bajorans, who all can breed. It turns out that this is possible because they all share a common ancestry, their homeworlds having been seeded by a progenitor race of humanoids in the distant past.
- Dungeons & Dragons:
- Planetouched races, such as tieflings, aasimar, and genasi, can usually be born to planetouched parents, but whether or not this is how they're usually born — and whether they originate from the interbreeding of humans and outsiders or from some other source — depends on what edition and what setting you're talking about.
- Forgotten Realms:
- Fey'ri, maeluths, tanarukks, wisplings and mur-zhagul are a group of minor races descended from various mortal species — sun elves, dwarves, orcs, halflings and trolls respectively — and fiends. Fey'ri in particular are highly proud of their bloodlines, and make a point of only ever breeding with other fey'ri.
- Worghests are descended from the intermingling of goblins and barghests, Lawful Evil shapeshifting Hellhounds.
- Eberron: Half-elves are considered a distinct race, as in most cases their purebred human and elf ancestors were several generations ago. There are even two Dragonmarked Houses composed of half-elves. One of the reasons there are enough half-elves on Khorvaire to make up a culture of their own is that when the elven island nation of Aerenal came into contact with humans on the mainland they expected that human and elves could not have children so many elves went to the mainland to marry humans as, essentially, an inheritance scam (being set to inherit their partner's wealth when the human inevitably predeceased them) — only to find out that not only could humans and elves have children, this trope was in effect for those children.
- Dragonlance: Since humans, gnomes, dwarves and kender are all descended from the same ancestors, all are capable of interbreeding. Dwarves and gnomes never do so, however, because of what happened when they did — the children of dwarves and gnomes are the gully dwarves, who are notable for being severely less intelligent and organized than either parent species (for example, they're completely incapable of counting past two). After seeing what they had begat, gnomes and dwarves vowed never to interbreed again, on pain of death; however, by this time, Gully Dwarves had become a viable race of their own, and have since maintained their own self-sustaining communities throughout the world.
- Much like in Dungeons & Dragons, tieflings and aasimar are the true-breeding descendants of intermingling between mortal humanoids and evil or good outsiders, although they are sometimes also born from the union of a half-fiend or half-celestial with a mortal, or from the resurfacing of long-buried traits in seemingly baseline mortal families. Genasi are replaced by ifrits, oreads, sylphs and undines, self-perpetuating humanoid species descended from humans and genies (and more rarely other types of elementals). In all these cases, self-sustaining populations can and do exist, but the rarity of all these races mean that in practice such things are fairly rare.
- Half-elves are sometimes Half Human Hybrids, but in some countries with a significant population they may be born of two half-elven parents. Th most notable in-universe consequence of this is Erages, a settlement consisting almost exclusively of half-elves within the elven kingdom of Kyonin.
Oppressed half-elves all across Golarion speak of Erages as a promised land, a place where our kind aren't "half-anything" — they're whole beings in and of themselves.
- The first fauns were the children of satyrs (Fauns and Satyrs being separate species in-universe) and particularly Good-aligned human women. Many are still born this way, but for the most part fauns have become a self-perpetuating species in their own right independent of humans and satyrs.
- Rune giants were originally created by the Runelords of Thassilon through forced crossbreeding of fire and taiga giants; they breed true with no particular issue, and have remained a stable independent species since the fall of the empire that bred them into existence. Likewise, slag giants arose through the crossing of fire and stone giants.
- Many half-dragons breed true and are capable of establishing themselves as self-sustaining species, if enough are created in sufficient temporal and physical proximity of each other. Dracolisks are the most notorious example of such half-dragon species.
- In Starfinder, orcs were almost entirely confined to Golarion at the time that it disappeared; as a result, most half-orcs now are the result of true-breeding between half-orcs. Self-sustaining half-orc populations are rarer in Pathfinder proper, but some — such as scattered communities in the Darklands and the island settlement of Averaka — are noted to exist.
- In Arcanum, elves and orcs are able to breed with humans to create half-elves and half-orcs, and the offspring of these unions are equally able to have children with one another. This is discussed in the manual's chapter on races, when an archeologist called Beddoes proposes a controversial theory that humans, not elves, are the oldest race on Arcanum, and both elves and orcs are descended from humans who were mutated by exposure to magic:
Beddoes: "The premise most difficult to accept is this: two organisms cannot create offspring together unless they are closely related. Although every naturalist and farmer knows it, we are reluctant to accept the same premise when it is applied to ourselves! It is nearly impossible to make the average elf, orc or human accept the truth: the very fact that half-elves and half-orcs exist at all must mean that the parent species are cousins to one another. Further more, the fact that both half-elves and half-orcs are fertile, viable hybrids, rather than sterile sports, means that the relationship between the parent species is very close indeed!"
- Diablo: According to the lore of the series, demons and angels are able to breed with one another to create hybrid children called Nephalem, who in turn are fertile and able to breed with each other. However, demon/angel unions are rare due to the two races being locked in a Forever War with each other, and the first generation of Nephalem (born from a truce between the angel Inarius and the demon queen Lilith) eventually became so terrifyingly powerful that a Power Limiter artefact was used to cause future generations of Nephalem to be born much weaker. Thus, the Nephalem became the ancestors of the setting's humans.
- The Elder Scrolls:
- The Bretons are a distinct race that resulted from an ancient Aldmeri eugenics program which mated their Nedic (the common ancestor of most races of Men) Breeding Slaves with the Direnni Altmer of High Rock resulting in humans with elven characteristics such as a greater affinity for magic and — occasionally — slightly pointed ears. They are occasionally referred to as the "Manmer", a name which acknowledges their elf/human ancestry. The word "Breton" itself is derived from the Ehlnofex word "beratu", or "half". Despite this, modern Bretons are still far more Man than Mer.
- The Bosmer (Wood Elves), are a sort of inverse of the Bretons. The Monomyth suggests that early in their history, the Aldmer who became the Bosmer "soiled Time's line" by "taking Mannish wives", making them a more-Elven version of the Bretons' Uneven Hybridization. It is worth noting that out of all the races of Mer, the Bosmer are the ones who look most similar humans and have the most human-like skin tones.
- The majority of half-elves in Tales of Symphonia are the children of two half-elves, from communities made entirely thereof. They greatly exceed the numbers and geographic distribution of the pure elves, who are only shown occupying a single Hidden Elf Village. Exire is a whole self-sustaining village of half-elves, most of whom have never even seen a human.
- Them's Fightin' Herds: The longmas of Huoshan are a species of sapient, winged and draconic equines who arose from the interbreeding of (also sapient) horses and dragons. The actual interspecies coupling only appears to have happened once, between the dragon and the stallion known respectively as Honored Mother and Honored Father. All modern-day longmas are the descendants of these two, and all are born from longma parents.
- In El Goonish Shive, this was thought to be averted for elves who are hybrids of the god-like Immortals and humans in this setting. However, this turns out to be a lie Immortals told themselves in order to spare themselves the attachment to extended families. In actuality, not only are elves true-breeding but all wizards have Immortals in their family tree.
- Star Wars: The Clone Wars: The Dathomirian species is the result of generations of crossbreeding between human women and Zabrak men, eventually resulting in a self-perpetuating hybrid species. They're also a case of Bizarre Sexual Dimorphism and Gender Equals Breed, as the Nightsisters look like extremely pale-skinned humans, while the Nightbrothers look like full-blooded Zabraks.
- Eastern coyotes are the result of cross-breeding between coyotes and grey wolves or domestic dogs, making them larger and more pack-oriented than their western kin and allowing them to move into ecological niches opened by the extirpation of wolves in the eastern states.
- Many species of small wild cats are capable of successfully producing fertile hybrids with one another. This has resulted in the creation of a number of domestic cat breeds created by breeding domestic cats with wild cats of various species and then breeding the hybrids together to obtain varieties of cat with appearances and coat colors housecats don't normally develop. Examples of stably self-perpetuating breeds created in this manner include bengal cats (descend from hybrids of domestic cats and Asian leopard cats), chausies (domestic cat x jungle cat) and savannah cats (domestic cat x serval), although first generation male savannah cats tend to be sterile, requiring the females to be bred with other domestic cats.
- The Clymene dolphin, a species of small dolphin native to large tracts of the tropical and temperate Atlantic ocean, has been determined through genetic testing as having originally arisen through the hybridization of the closely related spinner dolphin and striped dolphin.
- This phenomenon is somewhat more common in birds than in mammals. Both a species of Galapagos finch and one of manakin are known to have arisen in the manner. A more complicated relationship of this sort exists between the pomarine and great skuas: it is known that one originated from the other interbreeding with other skua species, possibly as recently as six or so centuries ago, but exactly which species came first and which arose from self-perpetuating hybrids isn't entirely clear.
- Though not confirmed, it has been theorised that the European bison (Bison bonasus) originated as a hybrid between an aurochs and a steppe bison, both of which are now extinct.