This is present in most unrealistic fictional genres, including Science Fiction (or, rather Space Opera), fantasy, horror, etc. In reality, many closely related Earth species share over 99% of their DNA, but cannot produce viable offspring. This makes it extremely unlikely that creatures of different planets would be able to interbreed, but then again, Mars does need women.
In fictionland, human beings can conceive children with any intelligent species in existence. Demons, elves, aliens, vampires, you name it not only will a human sleep with it, they'll engender children. The mother is in a majority of cases the human, with the non-human father providing the powers. If they are not physically viable, rest assured there are scientists somewhere who will fiddle with DNA until a hybrid is created. They'll do it even if there's no reason to think the creature should have DNA in the first place. That's Earth's solution for storing your genome. It requires a lot of supporting stuff in the cells, and not even everything on Earth uses it. Thankfully, the Rule of Cool and Rule of Sexy let us mix it up with whatever the Green-Skinned Space Babe's ancestors evolved to store their genomes in. Good thing there are No Biochemical Barriers! And if you can't find a scientist to do it for you, you're in luck. A Wizard Did It is just as handy a Hand Wave.
Sometimes a particular type of hybrid becomes common enough that they can stop relying on their parent races to make new members and just mate with each other to sustain their own race.
If the hybrid is a positive character, it connects the audience with them, giving them familiar characteristics and a closer point-of-view into the otherwise alien culture, or else, an "outsider" character for protagonists to befriend... or at the very least, they'll have the power to fight one-half of their heritage, almost certainly if that one-half is vampire.
Positive characters also tend to pick up all the advantages and powers of their component races with no disadvantages or weaknesses. A half-vampire might pick up Super Strength and Voluntary Shapeshifting without having a weakness to sunlight or a thirst for blood, for example, or a half-elf gets magic and an extended lifespan (funny, that). Being a Half-Human Hybrid is almost always a plus for protagonists, although the characters usually don't think so. Frequently it's because of self-image issues, possibly due to people getting freaked out at them having horns and hooves or the like. Females tend to be luckier in this regard than males, due to being seen as an exotic beauty.
If the hybrid is a negative character, this will make their inhumanity more personal. They might have suffered Half-Breed Discrimination, representing what prejudice on both species' part can produce. For extra angst, the character's conception might have been less-than-consensual.
The concept as a whole, thanks to A Wizard Did It, tends to fly better in supernatural settings — when it comes to, say, demons, who can say it wouldn't work that way? It's not like we can do a DNA test on the Prince of Darkness. Yet. In mythology and folklore, this is known as a "liminal being", like Merlin (who in some stories is half-demon) or centaurs. Also in mythology, for hybrids who are half human/half god, see Semi-Divine, and Nephilim, which are commonly portrayed as angel or demon/human hybrids.
For those people who turn into hybrids, see Animorphism and Mix-and-Match Critter. A character born human but changed to be part-nonhuman later on would be an Artificial Hybrid. If the character was already a different non-human species, or gets hybridized a second time, they become a Hybrid Monster.
Not to be confused with Little Bit Beastly and Beast Man, for characters that look like this trope but are actually a fantastical or extraterrestrial species that (for some reason) looks a lot like us with some added fur, ears, or a tail.
See also Interspecies Romance. A common result of a Fantastic Romance. May lead to Uneven Hybrids or Heinz Hybrids, if the family tree does not end with them. An inter-species humanoid hybrid that averts the "half human" part is a Non-Human Humanoid Hybrid. If the non-human parent is from an Always Chaotic Evil race, this trope may result from the human being Raised by Orcs.
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- Big Finish Doctor Who:
- Alex Campbell, from the story "An Earthly Child", was the son of Susan Foreman (the Doctor's Gallifreyan granddaughter) and David Campbell (human). Unusually for this trope, he is almost entirely genetically human note and doesn't have the Gallifreyan ability to regenerate.
- Ex-companion Bernice Summerfield conceived a son with a Kiloran (humanoid with canine overtones) called Adrian Wall while she was possessed by a soul sucker called Avril Fenman. The birth didn't go so well as he was too large to be delivered normally so Benny had to undergo a C-section while in a prison cell. After being saved, he was named Peter Guy Summerfield. Being half Kiloran he looked mostly humanoid but with a few furry patches, dog-ears and a wet nose. He was also stronger than all but the most augmented humans by the time he was eight. This became important when Braxiatel manipulated/hypnotised him into killing Jason.
- Dick Tracy: Moon Maid and Junior Tracy's daughter Honeymoon Tracy, who looked perfectly human to start with (to the point that when the Moon People started being downplayed, there was no reason for a new reader to suspect she wasn't). Shortly after the new Moon Maid was introduced, reminding the readers of Honeymoon's origins, her antennae grew in.
- The eponymous Arlo the Alligator Boy is half-human, half-alligator. His birth father Ansel is half-bird. There's also the reverse merman Marcellus, and the Beast, who's more like a dog-cat-tiger hybrid.
- The Little Mermaid II: Return to the Sea: Ariel's daughter Melody is half-human, half-mermaid, and wants to live in the sea, which is the exact opposite of what her mother wanted in the first film. In the end, she is given the choice to become a full mermaid, gain a tail, and live in the sea, but she declines, in order to reunite both halves of her family, and presumably spends the rest of her life switching back and forth from human to mermaid.
- Tuatha from The Fallen Gods is a half-elf. Amusingly this means that the party composed of her, human Flint, and elven Solvin, is exactly half-elf and half-human.
- The bard Mira Marchand of Trials & Trebuchets is a half-elf. other notable half-human hybrids include Mira's three half-elf sisters, the half-elf Elrich Raethran and his daughter Delnys, and the half-orc Kormet.
- Bat Boy, of Weekly World News fame obviously qualifies.
- This was already being parodied by the Victorian period: in Gilbert and Sullivan's Iolanthe, Strephon is half a fairy, "The upper half, down to the waistcoat", and worriedly ponders what he'll do when his lower half grows old and dies.
- Bat Boy: The Musical, which is based off the Weekly World News story.
- Elphaba from Wicked. Since it's shown in the very beginning of the story that her mother (the wife of the Munchkin Governor) was having an affair with a mysterious man who gave her a "green elixir" to drink, and Elphaba was subsequently born less than a year later with green skin, the audience could surmise from early on that her "father" the Governor was not her real dad. (In fact, he likely suspected it himself, as it's implied that this is one of the main reasons he always hated Elphaba.) However, The Reveal at the end of the play proves that she's this: her mother's mysterious lover, her real father, is in fact the Wizard of Oz himself, a human who originally came to Oz from our world. Elphaba being half-Ozian, half-human, a product of two different worlds, is surmised to be the source of her abnormally strong power.
- Amaluna: Cali, the half-man, half-lizard pet of Miranda.
- In The Ring of the Nibelung Hagen is half-human and half-Nibelung, being the Bastard Bastard son of the Big Bad Alberich. Apparently this leads to him being older then he looks.
- In Shrek: The Musical, Farquaad is half-dwarf.
- In A Very Potter Sequel, it is revealed that Draco's true biological father is Dobby, the Malfoy's house elf instead of Lucius, born after Narcissa had an affair with him while married to Lucius. This makes Draco half-elf, explaining many of his quirks played for laughs in this musical and its prequel.
- SCP Foundation: SCP-748 ("Industrial Dissolution"); one of SCP-748's products is the (highly radioactive) meat of a creature that is a hybrid species of human, squid and pig.
- New Vindicators has a lot of these running around-the primary source for superpowers comes from being a Neo-Sapien, which are all descended from Nephilim, who are half human, half fallen angel.
- There are, in the Protectors of the Plot Continuum, a number of half-human Agents. These come from a great many different continua. Especially the ex-sues.
- Shane Myers from Strange Little Band is half alien, half human but most of his colleagues don't know this. The identity of his alien father is important later in the story.
- Several characters in the fantasy webnovel Tales of MU. Most notably Mackenzie, a half-demon.
- Caleb "Half-Face" from Little Lenny Penguin and the Great Red Flood, tween-turned-eldritch.
- Elcenia: Rhysel is a halfblood, which is an interesting version of this trope. Though she, specifically, is half-elf and half-human, she would have the same characteristics were she 1/256th human or elf, hence "halfblood", since "half-elf" would limit the term to genotype rather than phenotype.
- Yet another joke◊ on the subject of the other half being assumed human. Half-elf, yes...
- In The Lay of Paul Twister, Sarah is half-human, apparently as the result of a magical curse. The nature of the curse is that what the other half is varies from one day to the next: she might go to sleep as a half-elf and wake up the next day as a half-ogre, for example.
- A complicated version in the French novel "Les Animaux Denaturés", variously translated as "You Shall Know Them", "The Murder of the Missing Link", and "Borderline". A scientific expedition finds a new species of primate called tropis that can be impregnated by both humans and apes. A businessman comes forward to basically enslave them since they're as suitable for hard work as humans, but he wouldn't have to pay them. In order to push the debate further, one of the scientists (artificially) impregnates a female tropi and kills the child, standing trial on whether he killed an animal or committed murder on a human. It's determined that the tropis are human, foiling the businessman, but as their status was determined after the fact, the scientist can't be retroactively condemned.
- In Chrono Hustle it is eventually revealed that Jack is a Demi-God. His mother was a Goddess, while his father was a Human.
- Limyaael's Fantasy Rants: Limyaael discusses these in her hybrid heroes rant.
- This blog post on Shakespeare Geek claims, among other things, that Shakespeare was half human, half Australian Frilled Lizard.
- The Wandering Inn: Ceria, one of the side characters, is a half-elf, who are despised in some countries.
- In Farce of the Three Kingdoms Sima Yan is the son of Sima Zhao and the fully human Lady Wang. Besides the freakishly long arms and hair that resembles tentacles, he mostly looks human, although he reverts to his father's language occasionally when upset. His brother You on the other hand looks exactly like any other human.
- It's possible but not specified that Sima Zhao himself is this, given that he is supposedly Sima Yi's son and no one questions that in the slightest. However, we are given no information as to how the hell Sima Yi fathered him.
- Critical Role has three in the main party, all of whom are of the half-elven variety. Vex and Vax are twins born from a union between an elven noble and a human peasant, and Keyleth belongs to a druid tribe comprised of many different races. Not surprising of course, given that this is a D&D campaign.
- In the sixth episode of Critters: A New Binge, it is revealed that the main character Christopher is the hybrid offspring of his human mother Veronica and the Crite President.