3×3 Eyes: the Denizens of Darkness are the series Demons and are mostly found in the Sacred Land when they're not roaming the world of men. They tend to have grotesque appearences and most of them blindly follow Benares in his evil plans, though the presence of people with demon blood implies that they can live alongside humans just fine. Many of them have Chinese names ending with Gui (demon, ogre, specter).
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.
The Godhand themselves are immensely powerful, god-like demonic lords that rule the Astral realm. They manipulate fate and realityto cause as much suffering as the world possibly can havewhile remaining in the Astral realm, since they are not physical. They can possess objects to manifest on Earth, but this manifestation is only about as powerful as most Apostles. However, once every 1,000 years they can perform the Incarnation Ceremony, to manifest in the physical realm as themselves. When they do this, they are every bit as powerful as one expects a demon god would be. Interestingly, just like the Apostles, every member of the Godhand were humans too, and their Crimson Behelit is a special Behelit that only appears once in 216 years. Instead of sacrificing one person the bearer holds dearly, the Ceremony of the Godhand demands the sacrifice of... well, everyone the bearer knows. The true source of these Behelits, the Idea of Evil, is the lord that rules over the Godhand and all of reality.
In addition, other demonic beings exist in the form of evil spirits created from the souls of those who died in the grip of negative emotions like fear and hatred (which happens a LOT in this Crapsack World), which like to possess animals, corpses, susceptible humans, and even the very snow itself in one case. These beings are drawn to Guts' Brand of Sacrifice and are always seeking to claim him.
Black Butler: demons eat human souls and can take on human appearance, and have keen senses much better than a human's. They each have their own personal sense of aesthetics that they operate under and will form contracts with a human in exchange for their soul.
Devils, on the other hand, are a race of sentient creatures hailing from the Underworld capable of using magic grimoires and speaking with mortals. While devils are vastly smaller than demons, they are arguably more dangerous since they are possessed of intellect. Oh, and they like to torture mortals for fun.
In Blue Exorcist all the Supernatural creatures of the Fantasy Kitchen Sink are referred to as demons, demons manifesting from things to the Western idea of demons to Eastern Youkai, gods of different religions and even in the image of human ghosts. This is because they come from a formless mirror World called Gehenna and one (and nearly ubiquitous one) of the only ways to manifest themselves in the human World is through Demonic Possession. Depending on their nature they can possess anything: humans, animals, corpses, machines and even particles, or they can be summoned via Ritual Magic. All demons fall under a specific attribute — Fire, Rot, Air, Earth, and Water — and are kin to one of the Baal, or "Demon King". While demons can be malicious, they operate under Blue-and-Orange Morality depending on the breed and exorcists are even willing to work with and summon them for their own purposes. Later is is revealed that the Demon Kings were all non-sentient concepts before they found their way into the human world where they all formed egos and identities of their own, the most powerful one of them all Satan the Anthropomorphic Personification of Gehenna itself.
Chainsaw Man's Devils are the embodiments of humanity's fear of a certain concept, whose power is proportionate to how much they fear it. Primal Fear Devils, such as the Darkness Devil, are the strongest. Devils are capable of regeneration by drinking human blood and have full control over their element. They originate from Hell, born with an instinct to hate and kill humanity. Their appearance is varied, and while most Devils are Eldritch Abominations, they look more human the closer they are to humanity; the Control Devil, Makima, is completely identical to a human. Fiends, like Power, are Devils who inhabit a human corpse, retaining some of their demonic appearance and power. Hybrids, like the titular Chainsaw Man, are humans who have made a pact with a Devil, maintaining their free will along with the Devil's power. Devils and fiends can be killed, but both reincarnate as devils into Hell (or into Earth if they were born in hell) without memories of their previous life. Hybrids, however, can resurrect so long as their contract is held. The only known way to completely eradicate a Devil is to be consumed by the Chainsaw Devil, in which case the concept the consumed Devil embodies is retroactively erased from existence.
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.
Delicious in Dungeon: Demons are actually Eldritch Abominations accidentally summoned from a dimension of infinite energy, and are Emotion Eaters which feed on desire. Dungeons were created to trap them Beneath the Earth, but they use trickery to lure adventurers into the dungeons where they can grow stronger by continuously feeding on their desires. If allowed to, they can eventually grow strong enough to tear their way to the surface and bring about The End of the World as We Know It.
The beings called "demons" in the English localization of Demon Slayer: Kimetsu no Yaiba are not western demons at all. They're actually Oni with a lot of vampiric traits.
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.
Members of the Noah family have been referred to as demons by Black Order members. Well considering that the earl has horns under his hats they might not be that far off...
In Dorohedoro, devils are magic users who have evolved into a new, more powerful form through a period of painful training. Their forms resemble twists on the traditional Big Red Devil, and are actually meat-suits being worn by their original bodies.
Dragon Ball: the franchise has several different kinds of demons, and even actual demon species. Many people, who aren't demons by default can become demons. For full details, see here.
Even when the Saiyan arc outed Piccolo as a Namekian, it didn't make him or his previous incarnation less of a demon, he was just a demon who was also an alien. Piccolo only ceased to be a demon when he became morally good. Prior to the Saiyan arc, it was established that those killed by the Mazoku demons (the most notable ones in the franchise being Piccolo's clan) didn't go to the afterlife, but their souls were forced to be stuck in the living world. This doesn't seem to apply to other demons, such as Buu (see below). When Piccolo killed Raditz and Goku, the fact that their souls were allowed to pass on was taken as evidence that Piccolo was no longer pure evil, as his previous incarnation/father was. Furthermore, when Piccolo was training Gohan, he commented that Gohan would make a great demon soon, referring to his failed plan to convert Gohan to his evil ways, further confirming that in Piccolo's case "demon" was a moral/factional alignment rather than anything inherent to his biology.
Piccolo Daimao was a demon because he was literally just the personified evil of the Nameless Namekian. This gave him all sorts of weird attributes such as the aforementioned afterlife shenanigans, having a ki able to erase Kinto-un, having a telepathic link to his spawn, and being able to choose the appearance of his offspring. While all other Namekians spawned from eggs just look like normal Namekians, Piccolo spawned Drum, Tambourine, Piano, and Cymbal, who resembled gargoyles, pterodactyls, and dragons. His only regular Namekian spawn was his final son and reincarnation, who is the current Piccolo.
The inhabitants of the Demon Realm (distinct from Hell; the Demon Realm is in the living world) ruled by Dabra are all demons with special powers. Dabra, in addition to being tremendously powerful in physical strength/speed and energy output, can turn people into stone by spitting on them, breathe magical fire, materialize objects, spawn magical swords and spears, create illusions, etc. His sister Towa can possess people and turn them into demons and she can time travel to change history, rather than creating new timelines. The Daizenshuu guidebooks state that Dabra and Towa are part of the "Demon Realm race"; it's unknown if all members of this race are necessarily demons or if it's decided based on status as was the case with Piccolo.
There are also the Makaioshins, the Demon Realm counterparts of the main universe's Kaioshins. They are the highest gods of the Demon Realm, but are far inferior in power to Dabra. Shinjin (Core People) from the main universe, who usually become Kaios, Kaioshins, or their attendants, are shunted off to the Demon Realm if they're shown to be evil. It is unknown if that makes them demons too.
Fighters under the mind control of the evil warlock Babidi gain a mark resembling an "M" on them and begin to display strange attributes. Spopovich, one such person, was said to give off no life energy (which the main characters can normally sense) and could snap his neck back into place after Videl broke it, like a zombie. Toriyama said in a recent interview that the M stands for "ma", which means "demon" or "magic" in Japanese. Yakon, another fighter under Babidi's control, is also introduced by Babidi as a "majuu", literally "demon-beast." Video games give them (or at least Vegeta) the title of Majin (é”人/"demon-human", rather than é”神/"demon-god" like Lucifer, even though they're both spelled the same way when romanized).
Devils are inhabitants from Hell, and Uranai Baba is able to bring one of them to the living world as her mercenary. Like Dabra's people, Daizenshuu 7 lists "Devils" as one of the races of Dragon Ball.
Enma Daiou and his subordinates are Onis and their job is administrating the after life. Enma Daiou decides whether a person goes to heaven or hell. He is not considered a demon by the series standards, because he's not evil.
In Z Movie 12, one of the Oni was corrupted by the evil energy of all of Hell's inhabitants, turning him into the incarnation of evil, Janemba. Janemba has the abilities to alter reality and connects the after life with the living world, bringing back thousands of dead people to life.
Pure evil beings regardless of their race and species can become demons. There are clans of those demons, with two of the most prominent examples being led by Piccolo Daimao and Lord Slug, both leaders being Namekian. For some reason, Lord Slug hated sunlight and wanted to blacken the Earth to get rid of it, while Piccolo had no issues. Possibly as a Call-Back to Lucifer.
While Piccolo Daimao's reincarnation was also born as a demon Namekian, he wasn't as purely evil as Piccolo Daimao himself. After Piccolo Jr.'s defeat by Goku, he didn't try for world domination and he seemed to become less demonic, as Kami noticed that Raditz and Goku, who were killed by Piccolo, passed on to the afterlife. The demon aspect was quickly dropped as the more fantastical supernatural elements were supplanted by more of a science fiction vibe.
One of the original Dragon Ball movies introduced a "Majin" ("demon-god"; not related to the Majin race or the Makaioshins) named Lucifer who resembled a stereotypical vampire. He and his gang wanted to destroy the sun with a giant laser because they hated the light.
Majin Buu is a pink blobby magical creature of immense power (magical, physical, and metaphysical) created/summoned by the warlock Bibidi, father of Babidi. He's a seemingly ageless Sealed Evil in a Can who is at the bare minimum millions of years old, and has destroyed countless planets and stars in his rampages. Aside from the standard Dragon Ball Flying Brick + Ki Manipulation power set cranked up to eleven, he can turn anything into anything else with his Reality Warper Candy Beam, regenerate From a Single Cell, heal people of various ailments (including decapitation or blindness), create clones of himself, rip holes in the fabric of reality by screaming, teleport, absorb people to take their knowledge, power, skills, and personalities, magically shrink objects inside the confines of his body (he has eaten literal cities worth of people without gaining a pound), survive in any environment including space, and more. Crossing over with Our Genies Are Different, as Buu's title, Majin, is often used in Japanese translations of Western works to refer to classic genies (the genie in Disney's Aladdin, for instance, was called this in the Japanese dub). He first manifests as smoke after being unsealed and he wears vaguely Arabic clothes, and his grey form resembles a ghoul. The Viz English translation of the manga localizes Buu's name as "Djinn Buu", while most other English localizations have left "Majin" untranslated.
Dragon Ball Online reveals that the pure good Majin Boo spawned a wife and had several offsprings with her, creating his own race of Majins. According to the manga, Majin Boo was created by Bibidi, but Word of Godretcons this by saying that Boo already existed long before Bibidi controlled him.
Makyans are a demon race from the planet sized comet Makyo, the most prominent example being Garlic Jr. They offer to let Piccolo join their clan, indicating that, like the original Piccolo, they're demons by alignment rather than biology.
Hildegarn from Z Movie 13 is referred as a Gen-majin, or "Phantom Demon". Steve Simmons' subtitles localize this as "phantasm." He was originally a statue that was based on a Demon God (seemingly not related to Buu, Dabra, or the Makaioshins- confusing, we know), but evil wizards turned him into a living monster. Fitting to the name Phantom Demon, he can turn into smoke to evade attacks.
People possessed by the Black Mist Water (used by Garlic Jr.) become demons as well, similar to what Babidi does. They act like a typical Zombie Apocalypse.
Earth has also its own demon creatures.
In Earwig and the Witch, it is implied that the Mandrake is a demon, a shapeshifting beast that has magical control over the house, has pyromantic capabilities and has control over a legion of bat/bird-like lesser demons. He also apparently sometimes has horns.
Fairy Tail: The series has two kinds of demons. One is the Galuna Islanders, who have human size and shape, can sprout bat wings, and are a peaceful race overall. The other consists of demons from the Book of Zeref, who are generally massive, have unique and deadly magic, and are extremely hostile to humanity. The latter group has been distinguished by the name "Etherius".
In Frieren: Beyond Journey's End, demons are a race of horned humanoids ranging from monstrous to nearly-human in appearance. They are extreme sociopaths, lacking any form of human emotions but displaying exceptional skill at feigning them to manipulate humans. Their ability to speak, act, and even their human-like appearance are seemingly a form of convergent evolution which allowed the demons to more effectively hunt humans.
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.
In Helck everyone who isn't a human or a feral monster are collectively called demons, which includes beastmen and elves. Ironically, the Demon Empire is much more cultured, organized and civilized than the Human Kingdom.
Jujutsu Kaisen: Cursed Spirits (or simply Curses) fit the bill. Curses are living concentrations of Cursed Energy, a form of magic that originates from negative human emotions, meaning they are literally born from human fear, hatred, pain and regret. Cursed Spirits come in all shapes and sizes, but are near-universally hostile to humans, ranging from near-harmless feral little critters, to sapient and actively sadistic man-eating monsters. They have traits from both spiritual and physical demons: on the spiritual side, Curses are Invisible to Normals in most circumstances and can take multiple forms, as well as possess humans. On the physical side, they can interact with their surroundings the same ways regular beings can and take up real space. Curses also have multiple possible origins: most Curses are born in places that contain too much negative emotion, while other more powerful ones embody certain concepts which humanity collectively angsts about (such as smallpox or the fear of volcanoes, to name a few), and some even come directly from humans, such as dead humans remaining "alive" after death as Vengeful Spirits, or as creations of human sorcerers. Only Cursed Energy can harm Cursed Spirits, so only Sorcerers and Curse-Users can kill (or "exorcise") them with their abilities.
In Kyo Kara Maoh! 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.
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 The Mystical Laws, demons in this movie are depicted as Always Chaotic Evil beings of pure darkness that spread turmoil by influencing humans to give into their worst desires and drag them to hell, on top of tricking humanity that there's nothing beyond their physical reality.
In Negima! Magister Negi Magi, 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).
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.)
In Puella Magi Madoka Magica The Movie: Rebellion, Homura identifies herself as a demon upon rewriting the Law of Cycles. Whether this title symbolizes her evil or her self-hatred depends on the viewer- although she certainly looks like one, with the black wings. By solely studying her, it can be deduced that their souls exist outside of their bodies as tangible items, much like magical girls.
Rage of Bahamut: Genesis (the adaptation of the video game) has demons and fallen angels. Demons can take both animalistic or humanoid forms, with male demons usually wearing thick armour, while female demons dress more provocatively. Lucifer and Azazel fall (ha,ha) under the fallen angel category, appearing as bi-shonen humanoids with horns and feathered wings. Demons are also shown to form different tribes that live in Cocytus and serve Lucifer.
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.
Rooster Fighter: Demons are eldritch Kaiju appearing from humans and even animals with The Power of Hate. An indicator if a human is become one is a bud on their neck, that bursts if they get angry. The monsterization can be undone in some cases. Then there are devils, shapeshifting humans who can control other demons.
The Seven Deadly Sins: The Demon Clan features a massive amount of demons of all kinds, with the most powerful ones being usually humanoids and able to manipulate darkness and hellfire. The ones employed in combat in the Holy War are divided in several power tiers, different in color and appearence: Red (huge, fat and ogre-like, the first type to appear), Gray (lanky humanoids with tiny wings and huge mask-like heads shaped like the Sun), Blue (bird-like, fast humanoids), White (tiny, imp-like mooks), Copper (gigantic brutes with dome-shaped heads), Green (caterpillar-like beings with muscular torsoes and arms in lieu of legs), Orange/Persimmon (fat, koala-like monsters with great sense of hearing) and Ochre (multiple-tongue tadpole-like monsters with a stinger tail). They are lead by the Demon King, who appears as a colossal, black-clad Odinesque figure.
In Seven Mortal Sins, there are two distinct types of creatures found dwelling in Hell. The lower types are monstrous creatures that torment damned human souls, coming in numerous different forms and levels of sentience. Above them are the Demon Lords, beautiful women with powerful magic that sprout horns as a mark of their rank in Hell. They function as an Evil Counterpart to the angels dwelling in Heaven, and clash over the souls of humanity. The most powerful Demon Lords are the titular Sins, who rule over the Circles of Hell and defend it against heavenly attack.
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).
Superman Vs. Meshi: The Para-demons have buglike faces (particularly the netted eyes), but humanoid bodies, and skin as dark as coal.
Ushio and Tora is much the same as YYH and IY in the sheer variety, although traditional Japanese youkai tend to feature. There are also other monsters from China and foreign countries, including a Vampire, Homunculus, the Djinn-like Iblis and, in an extra chapter, western monsters and demons summoned by an Evil Sorcerer.
In Welcome to Demon School! Iruma-kun, demons take a variety of forms, from entirely human-looking except pointy ears and a tail, to Oni-like, to Beast Men, to downright alien. They all, however, have things in common: They can all sprout bat-wings at will regardless of how they look, they can all innately use magic, and they at times enter an 'Evil Cycle' in which their negative traits and desires are amplified, and they have to find a way to let go of stress. Despite everything they're not Always Chaotic Evil (though it's indicated that at one point they were) and aren't innately aggressive towards humans, and in fact most demons think humans are a myth in the same way humans think of demons.
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.
YuYu Hakusho "Demon" seems to be a race of beings that live in the Demonic World. Demons don't die of age, but can be killed, and are shown to have souls that go to the afterlife just like humans on earth (except in the movie, where it is revealed they originally went somewhere else). Other than that, there doesn't seem to be anything they all have in common; they may appear with horns and wings, or as anthropomorphic animals, eldritch abominations, or be very human in appearance. Their powers vary similarly, with very few fitting into stock demonic abilities. Most of their powers seem like they'd be more at home in superhero fiction.
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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).