...creations of Fallen Angels (Christian again) or Evil Jinn (Islam) created to help lead humanity astray and or torment them note Technically Nephilim are their "creations" so this and the above are not mutually exclusive
There's no doubt about it. No matter what you choose, Your Demons Are Different.
If the source is Japanese, and the demons are really different, the reason may be simply that, in translation, the word "demon" is being substituted for a completely different word, youkai, which really doesn't have a correlating concept in English. "Faerie creature" is probably closer when it comes to traditional function in folklore (in fact, on the occasions where "fae," etc. is translated into Japanese, the term will probably contain the same "you" as "youkai"), but "demon" sounds more likely to kick ass, despite faeries being pretty damn scary.
And even if they don't use youkai, Japan still has a lot of words that get translated as "demon." Mazoku, Akuma...
In Beelzebub, the demons look more "human-like" than anything else. They're typically possessed of magical power, and humans can make contracts with them to draw on that power, although that's not always safe to the mind and body of the human in question.
Hana To Akuma takes the Judeo-Christian version of demons, and tosses them in a blender with Western vampires and shapeshifters. Their king's named Lucifer, they consider fresh human blood a delicacy, and can alter their physical forms with a little magical aid.
YuYu Hakusho by a landslide. Hell, most of the demons in that show are different from traditional Japanese demons.
InuYasha is a classic example of youkai that are translated as "demons," even though they don't have to be evil. The oni suffer this too, despite that youkai and oni are completely different classifications. Although some oni had the misfortune of being called "ogre" instead.
Ushio and Tora is much the same as YYH and IY in the sheer variety, although traditional Japanese youkai tend to feature.
Chrono Crusade has demons that were called "akuma" in the original Japanese, and are based somewhat on the Judeo-Christian, European tradition. ...Kinda. They have horns which draw in spiritual energy to fuel their powers (like regeneration), and many of them are somewhat humanoid, but their forms typically vary—there's a Cat Girl demon, the relatively human title character, some demons that appear more like insect monsters...oh, and they appear to be in control of strange technology. In the anime, it's somewhat implied that they are corrupted, fallen angels, but in the manga it's revealed they're actually aliens, whose mothership crash-landed in the Atlantic ocean hundreds of years ago.
In Mahou Sensei Negima! demons seem to just be powerful creatures that people summon to do stuff for them, generally killing so far. Demons themselves seem to be along the lines of a Punch Clock Villain and display no animosity, are pretty cheery when saying goodbye and are rather casual. Then again, they don't seem to mind the whole killing innocent people thing much, either.
The Magic World shows us some demons who have their own free will, and they're shown to be... well, mostly like other humans and humanoids really, except with horns and other demonic features. Some are good, some are bad, some work as announcers, and some work as bounty hunters who lament the fact that their fearsome appearance means the scary fights are mostly dumped on them.
It's also apparently possible to become a demon by using a lot of Magia Erebea.
As a Fantasy Kitchen Sink Negima has many beings that get translated as 'demon': Kotarou and Setsuna are half Youkai (also called Hanyou), the most commonly summoned demons are Oni (which could probably be called Ogres), there are Kishin (translated often as demon god) like Sukuna, Mazoku (sometimes called 'True demons') like Morborgran or Zazie and her sister 'Poyo' or even half Mazoku Mana, Akuma (devils) were shown to come in low level forms like Rubicante, or high level like Graf Wilhelm Josef von Hermann. Also the form Negi is changing into thanks to Magia Erebea is called Mamono (demonic being).
The demons of The World God Only Knows are natives of the Underworld, who appear near-indistinguishable from humans, apart from magical abilities, some odd quirks (strange dietary preferences, unusual hair colours, etc.), and have a minimum lifespan of several centuries. Later, it's revealed that these are the new demons; the old demons, their predecessors, found they could draw power from negative emotions, and corrupted living humans to strengthen themselves. The new demons revolted, overthrowing the old demons and setting up their own society.
The demons in Raideen are aliens, complete with cyborg monsters and robot mooks. It's implied that human legends of evil spirits were caused or influenced by previous wars with these creatures.
The demons in Devilman are ancient organisms who can fuse with other living things and inanimate objects to gain many abilities and powers from what they've fused with, taking them over completely. Thus they look like the chimerical creatures from ancient texts. They have psychic powers normally of varying strengths. However, human consciousness is lethal to them. But a person who's scared or otherwise loses control of their higher mental functions can be taken over. Should that person have a pure heart, however...they will posses the demon's flesh and gain their superpowers! And thus was born Devilman!. They also showed up in a Crossover movie featuring Devilman and Mazinger Z.
In Kyou Kara Maou the demons look exactly like humans, and are not inherently good or evil. They do, however, live five times as long as humans and have magical abilities.
Nanatsu No Taizai They're HUGE, practically ogres, are incredibly destructive, and have multiple hearts.
In Naruto, there are nine super powerful demons that are differentiated by the number of tails they have (1-9). They have enormous amounts of raw power in the form of chakra that greatly surpasses any average human ninja in the series (though there are some exceptional shinobi, like Kisame Hoshigaki, A the Raikage, and the Third Rakage, whose power is said to be on equal levels with a tailed beast). While thought to be just merely animalistic demonic monsters, Biju are highly intelligent. Even after millennia since the Rikudo Sennin's death, they still care for him since he's their father. And Naruto is The Chosen One the Sennin prophesied which would see the Beasts for what they truly are, good beings that deserve respect.
Then, late into the Fourth Shinobi war, we meet the Ten-Tails which is the primordial beast which all nine Biju came from. It is Above Good and Evil and just... is... and can control weather, shoot either a Wave Motion Gun or lob unholy nukes across entire countries - and wipe them out. When Naruto tried measuring its strength in Sage Mode he and Kurama in comparison (Kurama being the strongest of the nine) were in front of the planetoid that represented the Ten-Tails.
March Story: They're called Ill, possess objects, and once they kill someone with your body they will never let you go. This is March's backstory. She got (slightly) better
In Slayers, demons are known as "Mazoku" and are the progeny of Dark Lord Shabranigdo, who is the closest thing the setting has to Satan (in the loosest possible sense, since God here is Chaotic Neutral and the chief good deity is Shabby's equivalent). They can create more of their ilk, and are divided into generational ranks, from highest to lowest: Shabranigdo, the Mazoku Lords (Shabby's direct progeny), Generals and Priests (progeny of the Lords), Master Mazoku, Lesser Mazoku, Brass and Lesser Demons (who are so weak that they aren't even styled Mazoku). The lower their rank is, the less is their ability to take A Form You Are Comfortable With: Lords, Generals and Priests have no trouble assuming human form, Master Mazoku have Red Right Hands and everything below that always looks monstrous, like the traditional demons. They feed on negative emotions, and spread fear and suffering to survive; they can be driven away by love and friendship, trapped in pentagrams, defeated with Astral magics or Black magics that use the power of a more powerful Mazoku. There are also Half-Mazoku, who are mortal beings or dragons made demonic (Valgaav and Zuuma are examples).
It's only one of many theories, but in One Piece, some researchers believe that Devil's Fruit houses actual devils, and that users actual gain their powers from these evil spirits. A superstitious CP9 member named Jabra told of a rumor that suggested that these devils would fight if placed in close proximity of each other, possibly ripping their way out of whoever had eaten them in the process; however, Blueno (himself a Devil's Fruit user) contradicted this claim, stating that Grand Line scientists have noted that the phenomenon only occurs when one consumes two Devil Fruits. (Thus far, no-one in-story has been dumb enough to try this. Indeed, Jabra's story sounds absurd, because Devil's Fruit users fight each other rather often without anything like that happening.)
Demons in The DCUspeak in rhymes. Well, the higher positions of middle management do; one's position in Hell is partly signified by how well one can speak. The lowest ranks can't form language, the Lords of Hell speak clearly and fluently, and Hitman featured one demon several ranks below Etrigan who could only say, "I am Baytor!" In the DCU Hell, a "lower" position is the better one. When Demons improve their standing in Hell, they fall even deeper into the pit. So a promotion is technically a bad thing. Neron was "promoted" to Rhymer after he screwed up big time. And he got killed off. For now anyway.
Raven of Teen Titans is half-demon/half-human who must control her emotions lest her demonic side be set free.
We get to see more demons as the series goes on. Some, such as Astaroth, Ualac & Helldad fall into the same horned humanoid mold as HB, while others seem to be a twisted combination of various human & animal parts. One storyline in B.P.R.D. features a demoness whose true form has the head of a crow, the wings of a bat, tentacles & an udder with dozens of human-like breasts hanging off it.
Part of this variation is because the different artistic styles of Mike Mignola and Guy Davis (the main artist of B.P.R.D.). While Mignola's demons and various monsters tend to be based more directly on real life animals, Davis tends to have a more Body Horror feel to his work. In one of Davis's sketchbooks he even comments that he had to tone down some of the "tentacles and Lovecraftian bits" for one of his demons before the comic went to print.
Marvel Comics' Anti-Hero Son of Satan is, as the name implies, the Half-Human Hybrid son of a very powerful demon who claimed to be The Devil. Of course, Marvel later backpedaled on the whole "real, honest-to-badness Biblical Satan" issue, and which of the many powerful Marvel demons it actually is has been Retconned several times. For that matter, the very nature of demons in the Marvel Universe seems to be a matter of some contention; fallen or degenerated Physical Gods, leftover magical energy from the early days of life on Earth, holdovers from the Creation of the Universe… And of course, the fact that all of them lie their horns off all the time makes it no clearer. Their behavior also varies quite a bit. Some (Mephisto, namely) stick to standard "deals, contracts, and bargains" behavior and seem quite comfortable with that, others (Dormammu, Chthon) are active schemers who are very much a threat to existence, and still there are others (Nightmare) who can't be reasoned with, but don't exactly try and do anything crazy beyond general malevolence.
Demons in Necrophim are strange-looking humanoid beings who resent having to share Hell with the Necrophim, and want the angels to finish what they started with Lucifer.
The Hellspawn from the series Spawn are humans who made a deal with Malbolgia, the ruler of the eighth realm of Hell, to return to earth for either love or vengeance. Such people usually get sent back several years to a decade after their original death. They're infused with dark energy matter known as Necroplasm, which they have a limited supply of outside of Hell, and are bonded with a symbiotic armor that acts to protect its host. Hellspawn have inhuman strength, speed, movement and healing, and have the ability to use necro-magic (supernatural powers including teleportation, phasing, resurrecting the dead, necro-energy blasts, and much more). In addition, Hellspawn are able to "feed" off the evil energy of the world including people and "dark animals" (rats, bats, roaches, worms, insects, wolves, and snakes), and are also empathic beings able to sense the emotions of others whenever they choose. Finally, they can manipulate the elements and fly. A Hellspawn's time on earth is limited, and every time he uses his necro-magic, it brings the spawn closer to a second death and a return to Hell. The Hellspawn's job is to supply Hell with souls for its army by killing people with his energy, either innocent or evil.
Divangelic from Empowered is a pair of Conjoined Twins - her right half, Vanity, is a devil, but her left half, Charity, is an angel. The mind boggles...
Demon Spawns Series: In this series of Groj Band fanfics, siblings Corey and Trina Riffin are revealed to be demons. They can alter their appearances and have their own different Elemental Powers. However, they lose all of their demon powers for the day if they come in contact with holy water. They're also the children of Satan.
Both siblings are half-human. While Trina takes after her father, Corey is more like his human mother Amethyst.
Okay, it's not really a demon, but at the end of The Incredibles, Jack Jack's powers include setting himself on fire, turning into heavy brimstone, and transforming into the freaking baby devil. It's probable they are homages to a famous superhero with similar powers, i.e., The Human Torch, Silver Surfer, and The Demon. Or he could just be an Expy of the Super-Skrull.
Films — Live-Action
The Dream Demons in A Nightmare on Elm Street. They are ancient and serpentine in appearance, and hold lordship over all nightmares. They find the most evil human imaginable, who they will grant the power to turn dreams into reality. They chose Freddy Krueger, and made him immortal too.
The genie race in the Wishmaster series are largely merged with much of the folklore about demons. The Djinn are one of three entities made by God (the others are Angels and Humans) while demons are not stated to exist as separate beings, the wish-granting is identical to a Deal with the Devil since the Djinn's prize is the wisher's soul, and their home dimension is almost identical to Fire and Brimstone Hell, where the souls he collects are gathered to suffer eternal torture.
Ferriman in Ghost Ship is a wicked former human who became a servant of Hell because of his sins. He collects souls for his infernal masters, calling himself a "salvager". He doesn't make deals, and instead tries to tempt people into committing crimes out of the sin of Greed to damn their own souls, which he can then take "home" when he fills his quotum. He can shapeshift, recover from gunshot wounds, and mark ghosts to become his servants in death.
The Aash Ra Va of the Astral Dawn series function in the role of demons. The dark Earth spirits later take over that role after the Aash Ra Va leave the Nine Dark Worlds.
In Piers Anthony's Xanth series, there are traditional "minor" demons (tempt humans, collect souls, horns, tails, etc.) and the Greater Demon X(A/N)^TH, a vast cosmic being who powers the entirety of Xanth's magic.
The smaller demons are also not necessarily evil (though they usually lack souls). Males tend to be big bullies, although some like Beauregard can get along fine with mortals. Females tend to be more naughtily sexy than malevolent. And all bets are off should any of them receive all or part of a soul: it tends to make evil monsters into good guys. As for the big, cosmically powered ones, they are also not evil, although they basically view mortal creatures as insignificant ants, and use them as the pieces in their big universal chess game.
Dora Wilk calls all citizens of hell "hellians" and divides them into devils (fallen angels) and demons, who lived there before devils arrived and took charge. Both groups are super-strong and look like humans, although some odd horn or strangely-colored eye might come in play and demons have bristles instead of hair.
The title character of Isaac Asimov's "Azazel" series of short stories is just out to help people... but his poor understanding of humanity and rather incompetent human intermediary tend to cause it to fail hilariously. Though he may actually just be a Sufficiently Advanced Alien. Also, he's only two centimeters tall. And the Unreliable Narrator may be making him up.
In the Myth Adventures series by the late Robert Asprin, "demon" is just a corrupted form of "dimension traveler", meaning that the term takes in a wide variety of species - and that humans who travel to another dimension are also regarded as "demons" there. Some demons look the part, like the horned, red-skinned, goat-legged Deveels, but even they're just avaricious Space Jews.
In R. Scott Bakker's Second Apocalypse, demons are summoned by Functional Magic from the Outside. The cost is that after the summoner dies, the demon will torture him forever as a payment for having been forced to submit to another's will. Some people don't think the cost that high in a world where sorcerers are considered damned anyway.
In Jim Butcher's The Dresden Files (particularly Death Masks and Small Favor), the Knights of the Blackened Denarius are human hosts to fallen angels. Each host, after an initial temptation, took up one of the thirty titular denarii (the thirty pieces of silver given to Judas for betraying Christ). Each of the Fallen lives within its human host, who grants him or her various abilities such as Voluntary Shapeshifting. If a host is killed, the coin he or she holds comes out of the corpse, which reverts to human form, and attempts to catch another host.
They are specifically known as the Fallen though. "Demon" is a generic term for any being from a specific region in the Nevernever.
In addition, there are a variety of super-demons known as Outsiders that come from outside reality itself. These are considered so horrible and dangerous that any wizard who summons one (or even seeks out detailed information about them) is sentenced to death. Also, one of the Senior Council members, the Gatekeeper, has it as his job description to keep the Outsiders from getting in.
The demons in Steven Erikson's Malazan Book of the Fallen are any type of alien creature from an Another Dimension. There are many different kinds; the only commonality is that they can be summoned and controlled by sorcerers or alternatively bargained with. As a consequence, some of the demons seen in action are gentle souls who just want to go home to their old life as a humble extradimensional peasant.
Later on it is revealed that the demons are the ancestors of the Witch Species of the series. An ancient elven curse reduced the demon's fertility resulting in their offspring being weaker and short lived. These became the first generation of the witches.
C. S. Lewis' The Screwtape Letters is an exploration of what sort of society demons might have. The demons' physical form is not described (though when Screwtape gets particularly infuriated he becomes a large centipede, requiring him to fetch a secretary to take dictation), but Hell is sort of like a Police State full of Corrupt Corporate Executives - ordered under Satan, but still squabbling and infighting amongst themselves.
In the Colt Regan universe, Demon is a sort of catchall term for various types of extraplanar beings with whose traits vary wildly from type to type. Often containing one (or more than one) of the common traits listed above.
Nasuverse demons are rather unique in that they are described as an actual 'substance' in and of themselves. Unlike other phantasm creatures (e.g. unicorns), they are created from the images of humanity and exist only to draw in disruptions. Possessions are actually the demons' attempts to draw pain and suffering away, but being 'impossible' creatures, their manifestation generally causes harm and eventually kills the host.
Then you have all the cultural variants which may also be labeled as demons but are completely different, like the ''oni'' in South-Asian countries.
In the Incarnations of Immortality series, Demons can be created out of "ether" by the Incarnation of Evil and can look like pretty much anything he wants them to.
In The Descent by Jeff Long, it turns out that there is a physical, real-life equivalent to Hell and demons - an underground world of tunnels and the brutish "hadals" that inhabit it. These creatures are degenerate hominids with demonic features and a taste for senseless, incredibly brutal violence. They are also led by a kind of Satan.
Middle-Earth demons are Ainur (angels) who joined Morgoth in rebellion against Eru Ilúvatar. Being shapeshifting spirits with various areas of influence, they come in several types, ranging from relatively weak vampires and werewolves to the immensely powerful Balrogs (and possibly Dragons too), and Sauron who seems to be (mercifully) unique in sheer power level.
Despite "orc" deriving from an old word for demon, Tolkien's orcs are not demons of any kind (though it's implied that some of their commanders were orc-shaped demons) — rather, they were mortal creatures likely bred from Elvish and/or human stock. Real demons are far more powerful.
There are also rather odd beings of uncertain origin, like Ungoliant, a bizarre spider-monster whose nature is never explained. As all the evil in Arda was caused by Morgoth, she might even have come from somewhere else. Then there are the frightful, "nameless things" that "gnaw" the roots of the Earth, mentioned but never described by Gandalf.
There are two kinds of demons in the Night Huntress books, those in their own corporeal bodies and those possessing hapless victims. A possessor cannot be killed, as they simply jump away into the nearest living thing when their current host dies. Corporeal demons are vulnerable to salt and cannot cross thresholds without an invitation.
In Black Dogs, demons are incomprehensible creatures stretching out into unknown dimensions, able to be summoned and controlled by uttering their names.
In Quantum Gravity, demons live on another plane of existence, and Hell is not a physical place. Devils are best described as evil spirits, and are basically the opposite of everything that demons stand for. Demons make everything an art—you might not want to think too hard about that—and are very much for being one's self, regardless of what that is. Devil's push people to make sacrifices (of self) now for some vague reward in the future, e.g. the greater good, prestige. A demon with (a) devil(s) cannot behave a a demon, and becomes an imp; imps push people through/around Hell. Demons come in many colors, and every shade means something different. Humans cannot become demons, but any creature who can interact aetherically can. Zal, for instance, is born elf but is also a demon.
The Demons (or, if you're a Morindim, the Devil-Spirits) in David Eddings' The Belgariad and The Malloreon, are incredibly powerful beings who can be summoned by the truly reckless (or the truly stupid) to do their bidding; practitioners of this are known as magicians or wizards. The smarter magicians bind their Demons inside an illusion and protect themselves with pentagrams and other magical protections; as long as the Demon remains inside the illusion, it must do it's master's bidding, if and when it breaks out, the pentagram will (hopefully) protect it's master. The stupidest magicians just summon the Demon and try to force it into doing their bidding. They seem to come in an infinite number of forms, and have a wide variety of powers from the application of sheer brute force, to More Than Mind Control, and full on Demonic Possession (of a freakin' dragon, no less). They seemingly cannot be killed, and must be sent back to their own universe; this is a pain for the heroes, especially once two contenders for the role of Big Bad start using a pair of Demon Lords as their respective Dragons; unlike normal Demons the Demon Lords are consenting to work for their masters, largely to fullfill their own agendas. Hell, their home base, is alternately described as another universe, and a "state of mind"; they are said to work for a Satanic figure known only as "The King of Hell," who seeks to become ruler of the universe. It's worth noting that summoning a Demon never ends well; eventually it gets away and either eats you, or (in the case of the two Demon Lords) drags your soul into Hell and feeds upon it for the rest of eternity.
The Shannara series has two kinds: the genuine Demons, who are evil Faerie creatures, sealed up beyond the Forbidding by the good Fae, and the "demons" of Morrowindl, who are really just another form of Shadowen. And then there's the Jachyras: evil Faerie lunatics so mad that even the Demons thought they needed sealing.
In Sara Douglass' The Crucible trilogy demons are the offspring of angels and mortal women who are consigned to Hell to keep them out of their fathers hair and because the angels consider procreation shameful although some have avoided this fate and seek to overthrow their fathers rule of Earth.
The negative implications of demons are completely averted in the Dragaera series, which has a lot of Dark Is Not Evil present, as the term only means "someone who can be summoned" without any moral implications. Verra the Demon Goddess (the patron goddess of some of the protagonists and the mother of one of them) is called as such because she was formerly a slave to the Jenoine before taking her position. There's also a character called the Necromancer who is an undead demon summoned from a place called the World of Seven Doors, who thereafter serves Sethra Lavode and aids the protagonists. This probably also explains why a gangster in the series is known as the Demon- he's an assassin and can be "summoned" for the right price.
In Kage Baker's Children of the Sun series demons are actually air elementals (Their name for themselves is "Children of the Air"). They're not necessarily evil, but are very chaotic. They have a natural form, which can only be seen by those with magical sight, but they can assume (or be forced into by a magician) a physical form. If it's a chosen form then they can also dissolve it and their next physical form can be totally different. If forced magically into a form they are stuck in it until released or killed. If their owner has their true name then they can be returned to that form. If not then they are free.
The Age of Fire series has demen ("deep men") - horned, dorsal-spiked, carnivorous hominids who inhabit the more inhospitable parts of the Lower World. They're not really demons per se, but the similarities are probably intentional.
In the Riftwar Cycle, there are seven known levels of existence, with each being more dangerous than the one above it. Mortals come from the first level; demons are from the fourth and fifth (the overwhelming majority of demons featured in the series are from the latter). They are characterized by being incredibly vicious and living only for the id, and possess the power to absorb traits and knowledge from other living things they eat; there are several different "paths" a demon's evolution can take as it aborbs more powers, though they all seem to start out as imp-like beings, and will eventually reincarnate if killed. They are ruled in their own realms by Demon Kings, the most notable of whom are Maarg and Dahun, but can be summoned by sorcerers to the mortal realm (weaker demons are much easier to summon and compel- incredibly elaborate preparations are needed for an attempt to summon Dahun, and no attempt is even made to try and control him). A Kingdom Besieged had a young demon named Child as a major POV character though it turned out she is actually a dead human character reincarnated as a demon- exactly how it happened we don't know yet.
In Fred Saberhagen's Empire of the East and Book of Swords universe, demons are powerful evil spirits who used to be nuclear bombs. When a general global nuclear war broke out, the United States activated a device called Ardneh that literally changed the laws of nature within the vicinity of planet earth to make nuclear reactions much less likely, preventing the chain reactions that make nuclear bombs, and nuclear reactors, possible. While this saved the human race, it also led to the collapse of technological civilization, and made magic possible and real. In this new, magical, world, the nuclear bombs became the demons. Fortunately, Ardneh still lived to fight them.
In The Chronicles of Amber series, "demons" is the term used to refer to all of the creatures that inhabit the Courts of Chaos, other than the Lords of Chaos themselves (a distinction which gets a bit blurry at times). Some look like the traditional horned and scaly monsters, but they run the gamut from talking cats to mathematical abstractions (they are creatures of Chaos, after all). The one thing they all have in common in that they are all capable of Voluntary Shapeshifting to one degree or another. This is not so much a magical power as it is an evolved survival mechanism for living in Chaos, where the environment is frequently subject to change without notice.
In Lord of Light, the demons are actually natives of the planet who lived there before humans came. They are disembodied creatures with the power to manipulate energy, and can possess people by controlling their neural activity, which they love to do so they can experience the pleasures of the flesh that they long ago abandoned. They are known as Rakasha, after the snake demons of Hindu mythology, but when they choose to manifest, it is usually as a pillar of fire, not a snake. Their energy-manipulation abilities are similar to Sam's, but his are more powerful, which is how he earned the title "Binder of Demons".
The Book of All Hours - the Unkin. Humans that experienced a unique event in their life that allowed them to touch the Vellum underneath reality. In the multiverse inscribed on the surface of the Vellum, these meta-humans have long since taken up different roles, presenting themselves to mortal humans in different ways in pursuit of power.
In the web-novel Domina, demons are just humans who used the toy maker to add horns, fangs, and maybe a tail to their bodies. Although there are the succubi...
Sandman Slim has Hellions, which are the traditional fallen angels, kissi, who are the detritus of God's creation of the universe and feed on chaos and negative emotions and demons who are fragments of the Old Gods.
Felix Castor has demons of many stripes, ranging from the old standards (Asmodeus, Moloch, succubi like Juliet) to more mindless, rampaging things (The Naming of the Beasts sees something Juliet compares to a herd animal of Hell possessing a gymnasium). Like ghosts, they can be called up and exorcised by any number of means. And on rare occasions, it's possible for a human ghost to metamorphosize into a demon.
The Laundry Series uses "demon" as a catch-all term for some of the intelligences called down from the platonic realms. The books have so far featured unbound incubi (taking the form of an old lover), bound succubi (turn the host into a sex machine, but consume the souls of those they sleep with - and if not fed, they'll eat the host's soul), and lamia (reptilian kill machines). Most of the creatures are more akin to the Great Old Ones than conventional demons, though.
In the Children Of The Lamp series, demons apparently exist (one or two are encountered) and are apparently Fallen Angels, but are only vaguely described. The Evil Djinn serve as demonic stand-ins, and their three tribes (Ifrit, Shaytan, Ghul) are all named after demons or evil spirits from Arabic legends. The most powerful and evil Djinn in the world, Iblis, is even named after the Islamic Devil.
In The Warded Man, the corelings rise up from the Core each night, and they, and their remains if they are killed, are destroyed by exposure to sunlight. They are almost immune to normal weapons, but can be kept at bay and even killed by wards, though the offensive wards were lost until Arlen rediscovered them.. They are also divided into various subtypes, most of which are associated with an element. They have animal level intelligence except for the mind demons, who are dangerously intelligent and Always Chaotic Evil.
In the Night Watch series all demons are just Dark (i.e. selfish and egoistic, but not necessarily evil) magicians in their Twilight form. They can assume it at will when in "normal" world, and they automatically switch to it deep in Twilight. Since all magicians are born and grow up as humans, their appearance reflects the views of their society and Evil Makes You Monstrous is played straight. Thus teen delinquents in Dark Watch (or Face of Black Palmira) look like Big Red Devils from crappy movies.
Not all Dark Others become demons: witches and warlocks (the ones "connected with nature") always stay human, albeit the stronger they are, the older they look; Dark magician Edgar stays more or less human, which says a lot; Fafnir was a dragon; Anton describes one mook he killed as a humanoid lizard.
Then there are "lower Others" — generally not very strong in their narrow specialization, quite weak outside it and with a very limited choice over their form. One such creature mentioned in passing in Day Watch is a "beskud"note a creature from Central and Eastern European folklore — literally "miracle devil". Little info is given other than slitted pupils and triangular teeth. It is not elaborated if beskuds share lower Others' ability to convert humans to the likes of themselves. If vampires and werebeasts can do that, maybe so do beskuds.
Daevanote a kind of Central Asian demon or jinn is something else entirely — "a golem made out of pure energy". Very powerful, hard to control and near-indestructible, unless the magician knows a very specific spell. In medieval Middle East creating and controlling a daeva was a part of exam to the rank of Great Magician.
Devona or "daeva-possessed" has nothing in common with daeva aside from being Central Asian. This is a human infused with magic to be a better assistant to a powerful mage. The one we get to see is a master of combat magic and Obfuscating Stupidity. His introductory chapter ends with Heroic Sacrifice.
Demons in the Discworld series are inhabitants of the Demon Dimensions and are malevolent. Exactly what they're like beyond that seems highly variable, but it tends towards the Eldritch Abomination end of the spectrum.
In The Bartimaeus Trilogy "demons" are more like genies, who exist on a different plane and can project themselves as almost anything but have no real physical form. And they don't like to be called "demon" anyway; that's what the magicians call them.
In the Daniel Faust series, Demons are divided by Choirs of Sin, which influence their personality and powers. For instance, a demon from the Choir of Lust gets a corrupt cop addicted to her touch as if it was heroin, while people become so listless in the presence of a Sloth demon that they might starve to death instead of getting out of bed.
Doing evil for evil's sake is a pretty low priority for demonkind, at least the ones we've met so far. They're either out to satisfy their own ambitions (Nicky and Sullivan) or they're operatives in the endless cold war between Hell's feuding courts (Caitlin and Emma). They can be polite and even friendly — as long as they're getting what they want. Stand in their way, though, and the politeness vanishes in the blink of an eye.
The main villian of Fancy Apartments is a dark spirit named Vrotheus the Vile. Dangerous, powerful, evil, and with a great fondness for hexes and curses. Oh, almost forgot to mention: is also a large ham.
Despite Buffy the Vampire Slayer and Angel ostensibly taking place in the same universe, demons had a drastically different role in the two series, and indeed this meant that by the end of it things shifted drastically. In early Buffy, demons are Always Chaotic Evil - always - while representing the fears and traumas of high school teenagers. In early Angel, demons represent the downtrodden. Sometimes. Except when they're Always Chaotic Evil. By the end of things, "demon" basically just meant "extra-dimensional alien" (except for the ones who were from Earth but pre-dated humanity, so it was still pretty confusing...)
This is justified arguably by the fact that all living demons are actually hybrids, so while some might be Always Chaotic Evil, others might just be chaotic neutral. The only true demon we ever see is the Mayor after his ascension.
Well, there's also the "top surface" demon coming out of the Hellmouth in the season 1 finale and in "The Zeppo". We never see the whole thing, though.
It may be that more of the demons in Buffy were Always Chaotic Evil than in Angel because Buffy takes place in Sunnydale, a town built on top of a Hellmouth and specifically founded to be a feeding ground for demons, while Angel takes place in Los Angeles, which has a greater diversity in the evil to demon ratio.
The Scourge claim to be pure blooded, supported by the fact that a machine designed to kill anyone with human blood didn't affect them. Adding a bit of complexity to the True Demon dynamic.
The Scourge are about to meet Illyria in an upcoming comic miniseries, so we'll likely find out.
We do get to meet one of the Old Ones (Illyria), but only after she uses Fred's (human) body as a vessel. Her true form is only shown in an illustration.
We get to see Illyria actually take her true form in the sequel comic After The Fall.
Given that Buffy's demons and magic are frequently a metaphor for growing up, the demons going from appearing as Always Chaotic Evil to being more varied in their portrayals could simply be a metaphor for the realization that life is not always as black and white as we think it is when we're young.
Case in point, Clem. Appearing in seasons six and seven, he's a nice guy, with the exception of liking kittens.
Truth be told, a lot of demons in later Buffy episodes seem to be more Always Chaotic Neutral or Always True Neutral. Take Anya's vengeance demons for example. They're not explicitly malicious, and in many ways act like bureaucrats of the demon world. They have hopes and dreams and desires, and if it means they have to make a man's penis explode in the course of their jobs, so be it. Most of Anya's relatives at her wedding are actually quite friendly, and only turn on people once Xander's dad starts acting like a complete asshole. Really, like in Angel, the implication seems to be that demons just have different standards of goodness than humans do.
There are several instances where Angel kills a horrible looking demon monster only to learn that he was actually on the side of good, and assuming him to be the enemy. There are guardian demons, protector demons, empath demons, healer demons... Most look quite monstrous regardless of their profession.
In Supernatural, the demons, both the low-level ones and the high-level ones like Azazel, Lilith, and Alastair, are human souls that have been twisted by Hell's torments. Lilith being the very first human soul Lucifer (himself a Fallen Angel, naturally) corrupted.
Reaper's demons have horns and can shapeshift, but mostly look and behave a lot like normal humans. Some of them even want to bring down the devil for non-selfish reasons. By planting flowers. Don't knock it, it probably would have worked in about 400 years or so.
Charmed uses demons as the main antagonists for most of the series. Generally, the term "demon" is used for any magical person or creature who uses their powers for evil. Initially, many demons had odd, otherworldly appearances as well as coming in more human-looking varieties, but budget-cuts in later seasons led to ALL demons basically looking like humans with a penchant for either black leather or sharp suits. At times, warlocks are also treated as low-level demons.
Lost Tapes, despite being about mysterious animals, has a few supernatural demons appear in it. Mainly, the Hellhound and The Jersey Devil. It also features the "Dover Demon" - no one knows what the hell that one was.
In the Old Testament, demons like Beelzebub, Lord of the Flies, were the gods of other tribes in the near East. Beelzebub was a corrupted form of Ba'alzebul, or translated from Phoenician, "Lord of the Temple". Zebub means "flies" in Hebrew, so it was a kind of a Take That to old Baal. Didn't help that at least one Baal (there were a bunch of them; not surprising since the word just means "lord" in most of the languages of the region) had a habit of divination by watching flies. (hey, the Romans did it with birds...)
In Slavic mythology word Demon is used to refer ANY kind of supernatural creature, which is not God.
The English word Demon comes from the Greek Daimon (Latinized to Daemon), but in pre-Christian Greek Mythology the meaning was quite different. A Daimon is a spirit or minor deity that lurks in the background doing minor things gods don't want to bother with, or personifying concepts like Death, Sleep, Famine, Insanity, Piety, and Hope. Generally Kakodaimones were the bad ones, and Agathodaimones were the good ones. Daimon was also used for a sort of personal guardian "fate" or soul-double that accompanied a mortal through life. Sometimes a mortal could become a Daimon. However, this was simply seen as a different form of existence, instead of a remarkable change.
Dungeons & Dragons has always maintained differences in Devils, Demons, and Daemons. Mostly. There have also been numerous minor fiendish creatures, such as Rakshasas, Oni, Barghests, Demodands/Gehreleths, and so on.
In the original edition of D&D, there were only Demons, with no Devils. The most powerful type of Demon was the mighty Balrog (which became the Balor after the Tolkien estate complained).
In the first edition of Advanced Dungeons & Dragons, Devils are Lawful Evil and come from Hell, Demons are Chaotic Evil and come from the Abyss, and Daemons are Neutral Evil and come from Hades. They were given little to differentiate each other aside from how Character Alignment defined their behavior.
In the second edition, due to controversy about being Satanic, the terms Demon, Devil, and Daemon were banned from the game and renamed "Fiends", which were separated into the Lawful Evil, corrupting, tyrannical Baatezeu (Devils) from Baator; the Chaotic Evil, monstrous, bloodthirsty Tanar'ri (demons) from the Abyss; and the Neutral Evil Yugoloths (Daemons) from the Gray Waste, who are extremely manipulative and expert liars. The Planescape campaign setting introduced the Blood War, an eternal conflict between the Baatezu and Tanar'ri, with the Yugoloths serving as mercenaries and arms dealers for both sides.
Third edition restored the names Demon and Devil, but otherwise changed little about their role in the game. They also retained the Yugoloths (but did not go back to calling them Daemons), but downplayed them. The Blood War was still going.
Fourth Edition finally broke the tie between the two Devil and Demon categories in nearly all aspects:
Because Neutral Evil is no longer an alignment in fourth edition, Daemons/Yugoloths have been turned into Demons, but are Evil like Devils, so they fulfill the machinations of the Abyss.
In Eberron, Demons and Devils are not a huge part of the setting. The concept of the dimension-spanning Blood War is limited to a single plane, Shavarath, where the Demons, Devils, and Angels are all locked in constant combat. Yugoloths still presumably exist in Eberron's cosmology but are given little attention. The more important fiends are the Rakshasas, who are actually native inhabitants of the world rather than extraplanar beings, and they ruled the planet for millions of years before being ousted from power by the dragons. The setting also includes two other broad categories of fiend, Daelkyr (Humanoid Abominations who are thematically closer to Cthulhu and co. than traditional demons) and Quori (nightmare spirits who seek to preserve their own reality by controlling all mortal dreams, and already rule one continent through the original Path of Inspiration) which are not technically demons, but might be colloquially referred to as such.
In Ravenloft, Demons (and fiends in general) are rare and mysterious beings, which enter the setting by transposing their bodies with those of morally corrupted human beings. Once in the Land of Mists, they are unable to return home; many acquire additional powers by performing rituals in various domains. Demons are able to cross sealed domain borders freely, as their vast evil cancels out the influence of darklords in a small area around them.
Pathfinder follows the example set by 2nd and 3rd Edition D&D, minus the Blood War. It defines the actions of the three fiendish races by their motivations: Devils want to corrupt the minds of mortals, Demons want to corrupt the body, and Daemons simply want to kill. Devils want dominion, Demons want destruction, and Daemons want oblivion.
In Dragonlance, Demons, Devils, and Daemons exist, but they are all subservient to the gods of evil. Since the evil gods seem to prefer using their mortal servants to carry out any tasks they want done, the three fiendish races are rarely seen.
In Infernum, a D20 setting, demons are biological weapons created by Fallen Angels by crossbreeding themselves with Spawn (sort of mindless, soulless, proto-type lifeforms), then established into the nine Breeds (Artificer, Beast, Deceiver, Fiend, Hulk, Imp, Malcubus, Stalker, Slaver) by a long regime of surgery, crossbreeding, incest and black magic. Realising they were expendable in the eyes of their "fathers" they rebelled, ate all of them they could catch, chased the others clear out of hell, then took the place over. They torture souls to extract a substance called 'iliaster', which is literally meat, drink and fuel to them. And these are your player characters. Oh, and there's no real difference between 'demon', 'devil' and 'daemon' — "demon" is the name of the race, whilst "devil" is a title given to demons that have performed a notable and impressive feat and "daemon" is the lowest of the noble ranks, awarded to demons that display true mastery in some art or skill.
The RPG In Nomine has an incredible assortment of demons. All they have in common is that they're Fallen Angels, and every demon is a perverted version of the angel they used to be. The exceptions are the Lilim, who are all the children of Lilith and were never angels. While all (non-Lilim) demonic Bands are indeed perverted/corrupted versions of a Choir of angels, the vast majority of "young" demons have been created in Hell. The difference in outlook of a "Hellspawn" (who never saw Heaven and just knows the propaganda of the Demon Princes) and the actual, older Fallen Angels (who do, often painfully, remember their time in Heaven) is pointed out in one of the player character sourcebooks as important to keep in mind when roleplaying a demon. Still, even Hellspawn who never were angels in the first place can achieve redemption and become one of the Choir that corresponds to their Band.
All true demons are a type of spirit that is a part of a Yozi — Eldritch Abominations / dethroned Creators of the Universe. Each of the 23 Yozis has multiple souls with minds and wills of their own — at least 12, but the most powerful among them, Malfeas the Demon City, has 23. These are called Third Circle Demons. In turn, each Third Circle Demon itself has seven souls, these are called Second Circle Demons. The Second Circle Demons can create new species of demons, which are known as First Circle Demons. The First Circle demons can range from 24 fingered harpists who use time as a harp to beetles that swim through flesh and eat poison. Oh, and shape-shifting prostitutes are in there as well (they don't want your soul either, just some flesh to make a baby with way too many parents). Oh, and the major religion of the setting considers summoning them OK as long as you know what you're doing (and are a Dragon-Blooded, mortals shouldn't reach above their station, which is genuinely good advice on demon summoning in game).
And then there are the Infernal Exalted, who come from Solar Exalted shards warped by the aforementioned Yozis. While the Infernals start out as humans given great power with a few demonic touches and a First Circle demon's voice in their heads, they ultimately develop into something closer to a Yozi than a human. And they don't have to stay under their Yozi patron's control, either.
In Warhammer and Warhammer 40,000 daemons are created by the Chaos Gods, who themselves are the personifications of mortal emotions in the Warp. In Warhammer the Realm of Chaos is effectively Hell, but its 40k counterpart does a few things differently.
The Warp in Warhammer 40,000 is the source of all Psychic Powers in the setting, functions as the universe's "Hyperspace", and is where mortal souls go after death. The Warp is made of "psychic energy" affected by, and possibly created from, the emotions of sentient life. Eventually, particularly strong emotions gave birth to the aforementioned Chaos Gods, along with lesser daemons who act as extensions of their corresponding god's will. Daemons of a particular god will represent various aspects of its patron.
Thankfully, daemons are unstable in the Materium, and must pass through a Warp rift, be summoned via ritual, or find a host. Once they have crossed into our plane of existence, however, daemons are extremely dangerous, as they tend to be empowered by literal gods or warp reality around them. The trouble is that any psyker that has not been discovered and trained to control his abilities is susceptible to sudden, involuntary Demonic Possession.
Finally, one more way these daemons are different, is that as purely psychic beings they are extremely vulnerable to anti-psychic weaponry and cannot even touch blanks. This has allowed some factions to develop a few specialized anti-daemon countermeasures. Such weapons are rare, however, and More Dakka is often sufficient to kill Lesser Daemons. Of course, when all else fails, there's always Exterminatus.
The demons of Demon The Fallen are angels who were stripped of corporeal form and cast into the Abyss; a recent clusterfuck in the spirit world freed most of them, though some of them were summoned to Earth by mortal sorcerers, bound to inanimate objects, and eventually became Eldritch Abominations. A demon typically needs a soulless body (e.g., someone who's brain dead, a severely catatonic mental patient, a recent murder or suicide) to retain a presence on Earth; strangely, this is actually a humanizing experience for many demons, as they get to experience what humanity's like after millennia of abuse. They still retain many of the powers they had as angels (which range from controlling flame to reshaping flesh to overseeing the dead), but can willingly tinge these powers with "Torment," which makes them more destructive and more likely to screw with humans.
In World of Darkness: Inferno (the New World of Darkness take on the subject), demons are embodiments of sin, looking to spread the sin they identify with on Earth to increase their power. Some demons can possess humans, giving them access to special abilities deriving from their iconic sin, both literally and thematically. Demons dwell in Hell, a realm of sin incarnate. The ultimate origin of demons is unknown; some demons are ghosts and spirits who fell to corruption, but not all.
Mage: The Awakening adds Goetic demons (the mage's own subconscious desires and inclination toward sin given spiritual form and manifested in the real world) and the demons of Pandemonium (natives of a Supernal Realm entirely unrelated to Hell).
Demon: The Descent introduces another take. Unlike the spirits covered in Inferno, these demons are former agents of the God-Machine who broke their links either by accident, deliberate achievement of free will, or "system fault." One of the stated moods of the line is "Techgnostic espionage."
Demons in Magic: The Gathering are generally somewhat expensive to summon, powerful, and potentially a hazard to their summoner. (A classic example is the Lord of the Pit, which basically turns on its controller if not regularly fed other creatures.) Most of them fall into the purview of black on the game's color wheel.
There isn't that much fluff as to where they come from, except in the Kamigawa block, where Oni are a particularly evil type of demon that make deals with ogres. Kamigawa is one of the few blocks to have demons without a trace of black mana in them - there are two oni that are mono-red.
Outside Kamigawa, most demons are the bat-winged, clawed, fanged, gigantic monster kind. However, one early demon was the flightless, half-fungus Mold Demon.
In Innistrad, demons cannot be truly destroyed, only sealed away, and their inhuman desires and urges take physical form as little devils that spawn from them and wreak havoc.
Griselbrand finds out that demons aren't so indestructible when up against a Planeswalker.
Gamma World demons are an incredibly recent addition (As in, this current edition is the first to have them) to the game, basically being other-dimensional beings that can only appear in the world with the right mathematical equations. Some of the demons include a Creepy Child, a Sinister Minister, a demonic police officer that is an obvious homage to O Brother, Where Art Thou?, and a gigantic Poisonous Demon.
Demons in Reign — although exceptionally powerful magical beings — are of decidedly natural origin, hatching from eggs that form in the earth of the game world. They also get really freaky as they get more powerful, to the point that the most powerful can usually break off parts of themselves to act as independent agents, and are statted with the system the game uses for kingdoms and organizations, rather than characters. Also, since they hatch from eggs in the ground, the world's giant fleas made of earthen materials which suck minerals out of the earth tend to be full of them.
Nobilis features both Devils and Demons. Devils are Fallen Angels (the motive for their Fall differs between edition: in first and second editions, they held corruption to be the highest principle, the foundation of existence, while in third edition, they fell because they believed everything, no matter how evil, deserved love); Demons are the beings who have populated Hell since before Lucifer's Rebellion, and are on a much smaller (spiritual) scale.
Doom's monsters are invaders from Hell. They either take a monstrous physical form or they possess humans who become zombies, and some of them, such as the Cyberdemon, the Spider Mastermind, and the Mancubi, have cybernetic enhancements. Mainly the only way you can kill these things is with lots and lots of firepower, with rockets and energy weapons being used against the most powerful ones. In Doom 3, you have to use a magical weapon called the Soul Cube to take down the biggest and baddest one of them all. Likewise, in Doom 3: Resurrection of Evil, you have to use The Artifact, a weapon created by Hell itself, to kill off the guy who started it all. Oh, and he turned into a skeleton-dragon. Oh, and at the end, it makes you invincible, incredibly fast, and causes you to turn hell knights into Ludicrous Gibs with your bare hands.
The Daedra of The Elder Scrolls series are immortal entities from the realms of Oblivion, and the closest thing to Demons in Elder Scrolls lore. The Dremora, a specific race of Daedra, are the closest thing (aesthetically, at least) to traditional Demons. It is even Lampshaded in an in-game book that the word "Demon" is simply a mistranslation of "Dremora" that became popular with the general populace before anyone could correct it.
Also, many are pretty friendly, although it depends upon the Daedra Prince. Some are princes of Destruction, or Rape, for example (the habitation and minions of the former were portrayed, in Oblivion, as looking pretty much like a standard-issue hell) and then there are princes of dawn and dusk, madness, dreams, and so on. Also, because they are immortal, they have completely different concepts of good and evil. So Mehrunes Dagon (the equivalent Satan more or less) cant be seen as really evil, since his reasons for the Oblivion invasion could be far too hard for mortals to understand.
It might be better to think of the Daedra as a whole as differently moral... being quite immortal, the actions and fates of mortals are of fleeing interest. Summoned by a mortal wizard and forced to do his bidding? Being bound into a magical sword for a thousand years? When you can exist for millions, its of no more irritation than a human might experience with a few weeks in jail.
On the one hand, there are Eldritch Abominations referred to as the 'Old Gods' or 'aberrations' but never as 'demons' per se (even though they are just as evil and just as old as anything you might want to call a demon).
Some of these "true" demons were originally natural species, such as the Eredar, that were corrupted and inducted into the Burning Legion, others, such as Voidwalkers and Abyssals, spontaneously generated out of the twisting nether, or the void or other sources of powerful arcane and fel magic. Many demons in the legions are also cybernetic or entirely artificial constructions; like giant magic robots powered by demonic energy.
The mechanics of the games categorizes all NPCs into a limited number of groups that can interact with specific spell types. Demons, Aberrations, and Elementals are all distinct categories mechanically.
The Burning Legion are actually aliens who hail from worlds other than Azeroth, most of whom were corrupted by fel energy. The above mentioned Eredar aren't even a race of their own but are merely a faction of the Dranei. A few, such as the vampiric Nathrezim, seem to have been nasty from the start.
Sparda in Devil May Cry seems to be some sort of part-insect/part-goat humanoid thing. Also, Dante and Virgil are humanoid lizards in their devil forms, and just about every demon is completely different to each other. During the gameplay, you are supposed to fight minor demons and few of more "famous" ones appear as mission bosses.
Demons in Dwarf Fortress are physically an extremely varied lot - procedurally generated beasts (as are Titans and Forgotten Beasts), ranging from horrible amalgamations of mundane animals with uncanny features such as three eyes, composed of things like ash or snow or what have you (though immune to fire and temperature regardless), with terrible poisons that will do anything from causing mild itchiness to making your eyes rot - but they all fly and ALL want to rip you to pieces. And there's more of them in Hell than there are ants on Earth. Sometimes they hijack human civilisations by tricking the inhabitants into believing they're a god, or goblin civs by means of brute force.
Final Fantasy XI's demons are of the summoned-and-serve variety that serve the Shadowlord, although it seems they originated from Odin, a Celestial Avatar.
In the Diablo setting, demons are psychotic hordes sometimes created or altered by their leaders, the Prime Evils and the Lesser Evils. Even though they seem to have free will, they still do the bidding of their particular masters without question. They come in a huge variety of forms — from almost-human to green porcupines to the Blob to totally alien. In the second game, many enemies are not true demons, but creatures mutated by the forces of Hell. Killing demons primarily sends them back to Hell, which is the reason that the Soulstones were created — the angels needed a way to keep the Prime Evils from returning.
In the first game as well - the undead are not demons but corpses animated by the power of the Prime Evils, and the weaker creatures like Scavengers are mutated once-natural animals.
Infernal is a demon, or at least a half-demon, as well. His mother turns out to be Lillitu, the Succubus Queen.
And that's only the official demons in the game lore. Good luck trying to count all the types of demon and half-demon player characters running about.
Arcana Heart demons are pretty much like your traditional demons. You know, comes from hell, could get summoned with a ritual, has horns and wings and fangs... well, besides the fact that all the demons we've seen so far are generally good, cheerful sorts with the most evil one being the Jerk with a Heart of GoldMike. Oh, they're considered so much of an Unusually Uninteresting Sight amongst humans that they can legally get married to human women and receive jobs in the corporate world while their obviously half-demon children (complete with little wings and fangs) go to school and become friends with the local Miko (who will nonetheless note to said friend that there are certain rules against demons threading on their shrine). There is even one that is a nun.
In Guilty Gear, although the eponymous Gears are artificial magical creatures intended for human evolution, how they're made and treated, as well as being hunted down by a Christianity-based order, their general appearance and their One-Winged Angel forms, make them parallel traditional demons. They also can mate with normal humans and have half-Gear children (complete with Gear features and possessing a One-Winged Angel form).
Albion, demons are magical creatures deliberately created by wizards for use as living weapons by giving form to their fears. The list of fears they came up with for the game isn't very imaginative or sensible, as the only demons in the game are Fears, Animals, Plagues, and possibly Storms.
Demons in Dragon Age: Origins are denizens of the Fade that long to experience mortal life and are defined by different emotions: Rage, Hunger, Sloth, Desire, and Pride. To this end they frequently attempt to possess mortals, mages being their ideal targets. (Mages are self-aware while visiting the Fade and a possessed mage becomes a new creature called an abomination.) Some of Anders' and Justice's dialogue in Awakening hints that any spirit of the Fade can become a demon if their desires become too strong. This is likely the reason Justice is both fascinated and disturbed by the love that his deceased host Kristoff and his widow Aura shared — the desire for such a thing could very well turn Justice into a demon.
This is demonstrated in Dragon Age II, where Ander's hatred of Templars has turned Justice into a demon of Vengeance. Anders can let Vengeance take control of him as a sustained ability; he will do more damage in this state, but can't be healed.
Touhou can a be a bit complicated. To begin with, there are creatures called in the Japanese akuma which are translated as devil; additionally vampires are a type of devil. Oni may also be a type of devil; some are said to work in the Celestial Bureaucracy and others live in an abandoned hell underground. (Note that the Japanese word for vampire contains the character for oni). All of these have been referred to as types of youkai (but clearly not all youkai are devils). It gets more complicated with the existence of Makai (literally Demon's World). It's apparently not hell, but it's said to be inhabited by demons in the translation (This troper does not know what the Japanese word used in the manual is), which may/may not be different from devils. It should be noted that Makai is ruled over by an obvious Satan analogue but she is in fact described as being a God, and having created the place.
In Primal the inhabitants of all worlds except for earth are called demons. Only the wraiths and djinns have a real tendency towards evil, and even among them there seem to be some people who are quite okay. The udine and ferai are actually rather nice people, and none of the demon races look in any way demonic.
In the Shin Megami Tensei franchise, "Demon" is actually a catch-all term for any supernatural being ranging from goblins and fairies to gods; even angels are technically one specific type of demon (though they'd probably be very angry at you if you called them that). Demons in general are still very bad news for unwary humans, despite not all being horned monstrosities straight out of a Doom game. This goes back to one of the original meanings of the word, simply "spiritual being." Technically the original word was "daemon", but close enough.
Karel in Fire Emblem: The Blazing Sword is known as the "Sword Demon", but despite his name and bloodlust, he's actually a good guy, and later abandons it in favor of "Sword Saint" or "Saint of Swords", also known as "Kensei" in the original Japanese.
Might and Magic have had multiple types of demons and devils since the beginning of the series. The first were powerful hostile creatures with no apparent connection to the later ones in the same 'verse, while the second type, introduced in the sixth game of the RPG series and third game of the strategy series, was a race of invading aliens properly called Kreegan but called devils by mortal races because some of them looked like mythological devilsnote And who happen to be the reason why the Ancients lost contact with the local arm of the galaxy., with demons being simply one of their subspecies. The Ubisoft-verse has another story altogether.
Darkstalkers uses demon as a term for monsters that are extremely powerful.
In Dark Souls there are several varieties of demon, and all of them were birthed from the Bed of Chaos, which is what the Witch Izalith became after a magic ritual went wrong.
Clive Barker's Jericho is set in "The Box", a sort of nightmarish dimension which God created to act as a prison for a Humanoid Abomination called the Firstborn. Every once in a while, the Firstborn attempts to escape The Box by using gullible humans as it's pawns, and it requires seven powerful warriors skilled in the supernatural arts to seal it back again, but doing so takes the pawns, the warriors and a piece of reality with it. The pawns, the warriors and any unfortunate goons taken along with the slice of reality are trapped forever in The Box, and if- no, sorry, when they are killed in there, they are brought back as hideous, insane abominations by the Firstborn.
Zoophobia enjoys and invokes this trope heavily. To name just a few, Damian◊, Zill◊, these◊ and these◊ characters are all classified as demons, or at least half-demon. Yes, really.
And also the many, intricate and diverse classifications of demons.
Reynardine from Gunnerkrigg Court is a talking mythological fox who gained body-snatching powers and subsequently lost his original body. Now he exists as some kind of fiery fox spirit that can't survive outside other people's bodies, which stinks for everyone else because he invariably kills his host. (This seems to be the sole reason why Sir Eglamore calls him a demon.) His cousin is a Trickster-god.
Though it seems pretty clear that human as though she may be, Zimmy isn't just a human.
It seems that in Gunnerkrigg Court "demon" is simply a pejorative used by people of ethereal beings that are dangerous, or that they simply don't like. Renard doesn't identify himself as demon, nor does anyone who befriends him. Likewise, he's prone of accusing Zimmy of being a demon, but no-one else agrees.
The Order of the Stick is based on the Dungeons & Dragons rules, but gives it a little twist. The main fiendish character is Sabine, and debate still rages over whether she is a demon or a devil (see Tabletop Games above for the distinction). Haley couldn't tell which type of arrow (silver for devils, cold iron for demons) would penetrate her Damage Reduction, so she used both, and still isn't sure which one worked. OOTS also features the Inter-Fiend Cooperation Commission, an experimental alliance between Demons, Devils and Daemons, who seek to prove that the three factions can achieve more together than apart. Sabine (and now Qarr, who at least is definitely a devil) works for them, further obscuring the issue of what side she is actually on. As for why no one can tell what Sabine is, both demons and devils feature attractive, winged, female creatures. Sabine most closely resembles a demonic succubus (she can shapeshift, drains life force, and describes herself as an "incarnation of illicit sex") rather than a devilish erinyes. However, she was sent to be the concubine of the explicitly lawful Nale. The revelation that she works for the IFCC might finally be an explanation for this, as it would give a reason for a chaotic being to be serving lawful interests.
The incredible [and sadly concluded] webcomic Demonology 101 by Faith Erin Hicks had demons that walk among people, but have pointy ears and teeth.
Some of the demons in Sluggy Freelance come from the Dimension of Pain. Others can be summoned through the Book of E-Ville. Although it was implied the demons of the Dimension of Pain are less traditional demons and more like sentient creatures (human or otherwise) transformed by magic and a poweful mutagenic compound. The Demon King may be the only true demon in the entire dimension.
Demons in Heroes of Lesser Earth are of the summon via conjuring variety.
Demons in Kismetropolis are of the fallen angel variety.
Demons in Drow Tales are actually creatures from other dimensions that don't have a physical form (and usually appear as floating red lights or black shadows with eyes) untill they latch on to the aura of a victim and eventually take over their body. Some of them like Khaless or Umpus are intelligent, and can pass off as full fledged members of their masters' clans. Most of them however are just animals that attack anything they see without a summoner to direct them.
Demon Eater The demons are cannibals and only transform after they eat.
Zebra Girl: The whole plot revolves around the accidental transformation of the title character from a normal girl into a demon, complete with hooves, bat wings, prehensile tail and pyrokinesis. It's a lot less fun for her than it sounds.
In Chirault, "demon" is apparently a blanket term for a lot of different dangerous (or not) creatures. These include horned humanoids (like the protagonist, Kiran), cat girls (called Kyrions), emaciated frog-monsters, and titans made of mud and tentacles. Some of these are intelligent, and some aren't.
In Planes Of Eldlor, an army of demons are lurking just on the other side of the rift, a gateway to their realm.
Darumatha is a classic Miltonian demon in many ways, announcing itself from the start as "The Demon-Dragon of Broken Hours," proclaiming its desire to overthrow the existing pantheon and rule the cosmos in turn, and tempting mortals to do its bidding with offers of unlimited power, glory, and treasure and/or Hannibal Lectures. It is also a shapeshifting entity bound from of old by the elder gods, whose default form is a world-spanning metal (yes, metal) serpent and whose preferred alternate form is that of a beautiful young woman. Despite having many good points to make about authority, righteousness and power, Darumatha has turned out to be definitely Evil. However, the difference between it and "the gods" is not so much a category one as a matter of metaphysical history being written by the victors, as the former ruling god and his angels were pretty horrible people themselves.
People tend to call any magical and/or dangerous thing a "demon" as well, such as shapeshifters, but this is not as careless usage as it first seems, since the lines are a lot blurrier than in Western mythology: anyone or anything can ascend to a "higher" level of being, given enough willpower, ambition, ability or training, and in theory become a demon/god if not defeated first. It's not easy, and it's not as if there are scads of ascending supernaturals around, but as in Eastern lore, it's possible: not all supernatural beings are the direct and deliberate creation of a deity.
In Wapsi Square, demons live in a separate world outside of our time line, attach themselves to sentient host creatures, and act in an advisory role. Most people can't see them directly, and they just seem to be a voice in the back of their heads. However, people can become living doorways to the demon realm, allowing them to see and speak to demons directly, and communicate with demons that aren't their own. In addition, if such a person dies under traumatic circumstances, such as a car accident, the demons can take control of the dead body, and continue to live without the host. They usually follow Blue and Orange Morality, but they do have a very strict set of rules.
The Demon in Friendly Hostility used to live in Fox and Collin's refrigerator. Not only is he not evil, he works as a nurse!
Demons in The FAN are psychic parasites that are formed as a result of powerful negative emotions in the minds of individuals with sufficient magical or psychic energy. They usually grow larger as they feed off these energies and eventually take over the host. Demons can be enslaved and called in using summoning magic. So far, their only weakness seems to be an enchanted camera that can trap and banish them.
Demons in Goblins are ancient entities from the deepest depths of reality (read: Hell) who are able to gain sustenance by inflicting suffering on the souls of other beings. They used to acquire suffering to consume by invading the realm of the gods and choosing some lesser divine entity to wage war against; when mortals came along, demons found them less satisfying as a meal, but the fact that they were plentiful and easier to subdue means many demons have switched to preying on them instead (Thunt, the author, likens this to a bear raiding trashcans in a suburban area). The concept of a Deal with the Devil arose because a demon who can aquire ownership of a mortal soul can torture it for the rest of existence, providing an infinite source of food. However, only the most powerful demons are able to take souls by force; most have to persuade a mortal to part with it willingly.
In Adventure Dennis the demons come in a variety of shapes and sizes and apparently live in Demonsylvania, ruled by King Demonevil. They kidnap the mayor because of his battery disposal policies.
Devil Girls in Sinfest are created by being "bomfed" (no, this is not an Unusual Euphemism, it's more of a sound effect) by Satan. There are also other devils who may have been bomfed or may have been born that way.
Blue at least appears to at one time have been a non-Devil girl in "Divinity School" with a young Jesus ... possibly an angel-in-training who was later tempted/corrupted by "bad boy" the young Devil. Young Blue has neither horns nor a tail.
The demons of Wildlife can be summoned and bargained with, but they are notorious tricksters and fond of using the Exact Words of a contract to trip people up. Taking a contract with them and failing to keep your end of the bargain generally means forfiting your soul to them.
The sura in Kubera are pretty obviously demons, being powerful supernatural creatures of varying strength that oppose the gods. The most powerful sura, the Nastika, are basically demon gods—they're actually more powerful than the gods, the difference is that the gods can resurrect themselves infinitely, while the Nastika only get to live once.
In Hell(p) demons are Hell's government and look pretty human save for occasional horns, tails, and wings. Although in this world, anyone can have those and more. People generally dislike and distrust them, but some think they aren't all bad. Confirmed when one demon joins the team of human protagonists.
In The Antithesis, Demons only differ from angels by their phenotypes of black wings and dark hair. They live in the same world, but exist on lower layers of a planet that encapsulates Heaven and Hell, called "The Atrium". They do not possess the evil characteristics of the Judeo-Christian religion, but are actually a population of humanoid-looking beings, and their society contains moralistic values, political hierarchies (the highest being Lucifer, their leader/Military Commander/President, the second highest being the Archdemons) that formulate their Parliament, known as the "Obsidian Court". They are characterized with eccentric personalities, and their culture is embellished in art, literature, and theater.
In Tales of MU demons need to feed from humans to survive, and they have a craving for human flesh well beyond said need... and most of the rest is hearsay. They do have a strong association with fire, and have been shown to possess humans and tempt them. The story also has another class of Petting Zoo People called "yokai" who in Fanon sometimes fall prey to the translation problem mention above.
The Salvation War has several different variants of demon. They are fallen angels, distorted by their environment. They invade 21th century Earth as part of the apocalypse. It doesn't work. That said, even the angels and, by extension, God/Yahweh and his brother, Satan are more akin to Sufficiently Advanced Aliens, with all their magic explained through various levels of EM field telepathy and generating of massive bursts of electricity, not to mention "body not built for flying" able to do so through internal sacs of gas that give enough buoyancy for their wings to lift... But tend to catch on fire or explode when damaged too much.
On the online Spec Evo project A Scientific Fantasy, demons are a family of giant lemurs.
Jimmy Two-Shoes takes place in Hell, and the entire population is made up of demons. For the most part, they seem to have no special powers and are human in all but apperance. Word of God says that Lucius is a Reality Warper, but we never see it in action. Though there was that one time where he snapped his fingers and there was a change in location...
The Legions of Hell in Teen Titans fourth season are mainly composed of floating, mute fire-creatures. A similar but scaled-up version with legs was summoned three seasons prior. But Trigon is even bigger, has blood-red skin instead of solid flame, antlers, and four eyes - basically Diablo with mutton chops instead of a tail. Raven, his half-demon spawn, has grey skin, purple eyes and indigo hair.
In Gravity Falls, Bill Cipher is referred to as a demon. He doesn't look very scary, he's just a flat triangle with an eye in the middle, but he has the power to enter people's dreams and can cause a lot of damage.
In the Rankin-Bass version of The Life & Adventures of Santa Claus, we have the Wind Demons. We only see one (the Commander), but he looks like a cross between a bat and a mosquito, tends to cause bursts of wind before he speaks, and while not malicious, is the most vocal detractor toward Claus being made immortal.