"All they know is how to hurt and destroy and kill. Admirable when you need someone hurt, destroyed or killed, but what about the times when you just want them to sit down and shut up? Oh, FORGET that."
— Dark General Cobalt, Sailor Nothing, about the problems with working with Yamiko
The Kingdom Hearts video games name the trope, with the manifestations of the darkness that exist in peoples' hearts; the darker the heart, the more powerful the resulting Heartless, which explains why Sora's Heartless turned out to be a Shadow; a lowly mook. They mindlessly seek out other hearts to steal (and thus increase their ranks), and tend to allow themselves to be directed by those who have control over darkness, or simply by those powerful enough. However, they were described as starting out as decidedly less destructive before the series' Big Bad, Xehanort, started his experiments on them, and created the Emblem Heartless.
And the Unversed, which are the 'opposite of human life', are born from raw darkness. They apparently feed off of negative emotions, in a different way than the Heartless do.
There's also Vanitas from the same game, who is the darkness from Ventus' heart, removed and made into a separate entity. The Unversed simply extend the process; each is a living incarnation of one of Vanitas' emotions, so, effectively, they were the Heartless of a Heartless (in terms of this trope's definition). Once Vanitas was merged back into Ven, however, they ceased to be.
Subverted with the Dream Eaters of Kingdom Hearts 3D. Dream Eaters are the naturally occuring manifestations of Darkness that exists within Sleep and Dreams, and they are split into two kinds. While Nightmares are bad and devour happy dreams, Spirits consume the Nightmares.
Anime and Manga
Sailor Moon's Nephrite eschewed using typical Energy Absorption tactics, preferring to concentrate dark power into a single highly motivated (and increasingly obsessive) person to get the best energy. His predecessor Jadeite worked in a similar way by amplifying negative traits in people, except he worked with large groups of people—Education Mamas, obsessive dieters, etc.—instead of just one person at a time. Zoisite and Kunzite just possessed people.
There is also Phages in the final season, and the Lemures of the previous forth. The former are humans without a 'Star Seed' while the latter are appearently people without their dreams (in the anime version).
The Big Bad of the entire series, is a God of Evil born from the malice of all living beings called Chaos, and he ends up having his essence scattered into the hearts of everybody across the universe... to defeat him. Chaos also spawned the Big Bads of all the previous four seasons, being lesser Eldritch Abominations compared to Chaos.
The Hollows from Bleach are fallen ghosts who have lost their hearts, and just have a hole in their chests, hence the name. The Shinigami 'heal' these creatures by destroying them with their swords, freeing the soul to move on to its final reward, while the Quincy destroy them completely (and believe they are justified, since Hollows attack humans).
Later in the series, Hollow are revealed to be thought of along the lines of intelligent beasts, unable to really do other than as their instincts tell them to. Arrancar are different and actually possess sapience as well as more positive emotions, leading to some of them being sympathetic characters, and a few of them even being downright nice. They're still more likely to be evil than good, though.
Still later, it was revealed that their heart isn't destroyed, but transformed. Rather than disappearing, the negative feelings in it cause it to become the Hollow's mask, and acts as the source of the Hollow's energy. Perhaps part of the reason an Arrancar is more human is that by destroying their mask a part of their human heart is restored.
A repeated, enigmatic symbol — particularly suggestive shadows — in the movie version of Revolutionary Girl Utena hints that Anthy is missing her heart. However, in an inversion, it is because she is the victim/captive of a monster, not the monster itself. This (probably deliberately) echoes the line about "a doll without a heart" in the TV series.
The Zonder, in the Humongous Mecha series GaoGaiGar are a variation on this; they don't feed on or arise from negative emotions in general so much as they feed on stress specifically. The forms they take on when they transform into Robeasts typically have something to do with the source of their hosts stress, and destroying them leaves the victim feeling quite mellow.
Yatagarasu, Shiho's Child, is described as residing in the darkness of the human heart in Mai-Hime.
Akuma in D.Gray-Man are souls of the dead who have been bent to the will of the Millennium Earl. Often grieving for a lost loved one the bereaved person calls their soul down into an Akuma at which point the soul of the lost loved one is imprisoned and has no will of its own. The Millennium Earl orders the newly made Akuma to kill the person that made them an Akuma and wear their body. So they're sad and in pain and become increasingly tortured and tormented as the Akuma develops its own personality around them. The main character, Allen Walker, has a special eye that allows him to see these trapped souls, and aims to release them from their pain... which means killing them. The rest of the Exorcists kill them just because they're monsters, and most of them wouldn't understand or take kindly to Allen's sympathy for them. It's not that the Exorcists don't know how the Akuma are made, but rather that they tend to forget, because they're not constantly seeing it.
One of Noah claims that if an Akuma self-destructs, rather than freeing the soul, the human soul is destroyed. She then demonstrates this to Allen, who watches in horror as the soul is torn into screaming pieces.
More generally, it's stated that the human soul will be released only if the Akuma are killed by an Innocence weapon. Any other method will destroy the soul. It's just that since normal humans don't have a chance of killing an Akuma in the first place, the question never comes up.
Miss Michiko from Dennou Coil, while not technically a monster, is born from Isako's rage and hatred.
In King of Thorn, most of the grotesque monsters came to existence as embodiments of the deepest, primal fears of scientists infected with the Medusa virus.
At a metaphorical level, Knives in the Trigun manga fulfills the definition too, as he might be viewed as an embodiment of the worst 'sins' of humankind. It is significant that in Trigun Maximum, he starts of as a loving and philanthropic kid and grows extremely fast into a crazed sociopath as a result of trauma or, more symbolically, of absorbing humans' violence and hatred. As such, he fits the Shadow Archetype not only for Vash but also for humans.
The Zero-Tails from the second Naruto Shippuden Movie is a demon made from the hate or other dark thoughts of others. It can also possess people feeling these emotions.
As part of Naruto's test at the Falls of Truth, the hatred that exists within Naruto's heart was given form for Naruto to confront it. He beat it by hugging it.
Also at a metaphorical level, the homunculi from Fullmetal Alchemist may be seen as embodying the 'darkness' or despair in the heart of the alchemists who created them and whose memories of a lost loved one they often carry. Or even of the original person or humankind itself. Their Theme Naming following the Seven Deadly Sins helps this interpretation. In the manga, they are manifestations of Father's cruelty and sins - what he is and whether he could be The Heartless himself is up for debate, though.
The X-charas in Shugo Chara! can be seen as a form of this: Usually, when a child needs help in fulfilling their dreams, these dreams materialize and become a "Guardian-chara". However, if the child has already GIVEN UP all hope, the dreams instead get corrupted with lack of self-esteem, the feeling of being useless and other negative-emotions and become an "X-chara" instead: A little, black creature, determined to destroy everyone and everything around it with the power of the dream, it was originally supposed to embody. The owner of the X-chara usually falls in a comatose-state, with his or her inner voice mourning about how useless he/she is. Amu usually tries to heal such Charas, by using the powers of the humpty-lock, while others, like Ikuto and Rima (at least in the beginning) prefer to simply destroy them — turning the owner into a child without any hopes for his/her life.
The Idea of Evil, the God of the Berserk universe, is created from mankind's desire for an explanation for why we suffer. Qlippoth creatures are also reflections of humanity's darkest side.
In addition, there's the Hellhound, Guts's personal Heartless and Superpowered Evil Side, which was born from all the negative emotions that arose in Guts after the Eclipse went down, and which worked in concert with him during the Retribution/Conviction arc, but now works against him as his focus has turned to protecting Casca.
Shikabane in Shikabane Hime are formed when someone dies with regrets or an attachment to this world, which turns them into a monster.
The emotional spectrum entities in Green Lantern are creatures born from, and embodying the emotions of sentient beings. Naturally, the ones born from fear, greed and rage are particularly nasty.
Despite the whole Zombie Apocalypse motif, this is what the Black Lanterns from Blackest Night really were- the unresolved emotions of the dead given form; they just pretended to be the actual (zombified) person in order to trigger emotions in their victims, which they fed off on. The dead person's real soul was not involved, and if the person had died in peace, they couldn't be duplicated.
Nega-Scott in Scott Pilgrim (comics only; the movie version is completely different).
To explain: In the comics, Nega-Scott was basically an Anthropomorphic Personification of Scott's repressed memories and his denial to admit he'd been a Jerkass since he started high school. Scott doesn't really "defeat" him, rather, through revelation of his true high school past through Kim (which was altered into him being an eternal moral do-gooder by both his repression and Gideon's tampering), and through contemplation during Nega-Scott maiming him, Scott accepted his own faults and absorbed Nega-Scott back into himself. In the movie, it's just a Giant Space Flea from Nowhere that Gideon sends to fight Scott at the end, which backfires when they become instant friends.
The main villain of one Aquaman story arc was "The Thirst", a golem which is animated as the shadow self of the chosen waterbearer should they use their power to harm rather than heal. His only desire is to drink life itself. They take this role to the waterbearer, their strength proportional to the bearer's negative emotions. Only after Aquaman finds it in his heart to forgive his archenemy can he muster the strength to defeat him. And when he's then betrayed by said villain, the response leads to him doing a Fusion Dance with The Thirst, leading from a fake ending into a far worse scenario.
Mephisto's "son" Blackheart was born from the evil of Christ's Crown (named after the crown of thorns used to torture Jesus), a place with a dark and bloody history.
However, in the original book, it's implied to be more a result of increasing cynicism and loss of imagination, as it's spread by people lying.
Hard to see how people lying, i.e. making stuff up, equals loss of imagination.
It's because being lied to sours people on the idea of things that aren't real.
The book is big on emphasizing the difference between lies (things made up to deceive), and stories (things made up to entertain and inspire). It also implies that lies are themselves the Heartless, being the dark side of creatures from Fantasia. Any creature from Fantasia who is caught by the Nothing is transformed into a lie and taken to the real world.
The truth is much worse than that: as it turns out, they're really Eldritch Abominations spawned from the Abyss, the great void of nothingness between the universes. They devour the souls of unfortunate people that happen to be near them. And they're also created whenever the Subtle Knife is used to cut between the worlds. And they destroyed civilisation in the world that originally created the Knife. Oh, and you can't kill them by physical means.
Harry Potter: Dementors are a borderline case. We never learn how they come to be, or, indeed, if anyone knows this at all, but their presence drains the positive emotions of everyone around them, and they will swallow the souls of anyone too weak to fight back if given the chance. The Ministry actually uses them as prison guards, although some (such as Dumbledore), question the wisdom of this approach - with good reason, as they defect to Voldemort almost as soon as he asks, since he doesn't impose any restrictions on them.
As are boggarts, whose shape depends on what the person nearest them is most afraid of.
The series also does something of an extension on the the trope by having the Infernal Devices (swords) chosen to fight the demon armies capable of burning the humanity right out of the user, in essence making them the same or worse than the things they're fighting.
In Neil Gaiman's American Gods, Odin thrives on death and Loki on chaos. Shadow thwarts both of them by standing before the impending war of gods and explaining exactly that. The gods go their separate ways, and the death Odin was counting on to resurrect him evaporates.
Hyde could be seen as the Ur-example of this trope. Also unmentioned above but relevant to this trope and the quote, the story not only makes much of Hyde's utter lack of any kind of positive emotion or morals, but also points out that because he has none of these things, and hence no limits, he has an unrivaled love for life. It is this addictive sensation that actually draws Jekyll into continuing the experiment until it is absolutely too late to turn back.
In the second volume of the League of Extraordinary Gentlemen, Mr. Hyde discusses this with the others; he explains that he's been getting bigger over the years while Jekyll's been wasting away because Jekyll kept all the restraint for himself.
The Beast in Lord of the Flies is initially thought to be a physical threat, but eventually we come to understand that it is the darkness of man's heart, and is the corruption of human kind's minds. Though the Beast is never actually seen because it's all in the characters' minds.
They do, however, mistake other people for the beast, one being a dead airman who has ejected from his aircraft above them and the other being Simon, who they kill and who Golding gives a beautiful burial.
In "The Fisherman And His Soul" by Oscar Wilde, a Fisherman cuts his shadow (which holds his soul) free from his body so that he can live in the sea with his love, a mermaid. The soul, lacking a heart, becomes evil.
The Serpents in The Death Gate Cycle were created by magic gone mad fused with their creators' fear and hatred- their goal is to turn the universe into "a prison house of suffering and death" because negative emotions give them power and prolong their existence. Unlike a lot of examples they're actually very smart (most of the time), and can take whatever shape they want to further their goals, though they prefer appearing as hideous snakelike creatures in order to best terrify people.
The Overlook Hotel in The Shining is depicted as being The Heartless in architectural form, if you go by the interpretation that the paranormal side of what's happening in it is genuine.
The Shadowen in The Heritage of Shannara are very much this trope: smoky, bodyjacking wraiths made up of people's negative emotions and hatreds.
From the same author, the feeders in The Word and the Void are creepy, shadowy beings that feed on negative emotions, and induce them in people to boot. The demons, on the other hand, are closer to The Soulless.
The Fearlings in The Guardians of Childhood , shadowy wispy spirits that spread fear into the minds of children by turning their dreams into nightmares and convert them into their ranks if overdosed.
The Voices from Of Fear and Faith are powerful demonic monsters created by humankind’s negative emotions that instill these emotions on anyone who gets near them.
Live Action TV
Red Dwarf makes ample use of this trope in many episodes, such as:
In one episode, the crew land on a moon that forms itself to mimic the mind of Rimmer, who is... all the worst traits of humanity. They are trapped, and decide to build up Rimmer's self-esteem in order to escape. This culminates in giving Rimmer a group hug.
"Polymorph", which featured a monster which feeds on emotions such as fear, guilt, rage and vanity, and changes its shape in response to each person's inner demon in order to elicit these emotions.
Rimmerworld, which is populated entirely by descendants of Rimmer who have weeded out any trace of bravery, kindness or loyalty, in favor of all of Rimmer's worst qualities. They're all named Rimmer.
Legion, a "gestalt entity" comprised of the four crew members' combined consciousnesses. They defeat it by knocking themselves unconscious.
The Mara from Doctor Who. It is a parasitic being of pure hate and rage that requires the fear of its victims to survive. As the Doctor learns from a mystic in its second appearance, "fear is the only poison.". When forced to reveal itself, it looks like a rather pathetic giant carnival snake.
The Valeyard. The Doctor's worst possible future self, created by the Time Lords to condemn him. In return, he would get the Doctor's body. He's so bad, he terrifies the Master, who once held the universe to ransom.
The Dream Lord. More a Master of Illusion and personification of the Doctor's darkness and self hatred than the Valeyard.
The creature Armus from the Star Trek: The Next Generation episode Skin of Evil describes himself as having come from a race of Titans who learned to physically manifest the dark part of their natures as a second skin, which they then discarded. Armus is the amalgamation of all of the discarded evil, or potential for evil, of a race. To say the least, he's a bit peeved about being left behind.
An interesting example in that Armus is defeated by confronting him with that fact that he is less heartless than he first appears.
Ultraman 80 had a whole series of monsters created by negative emotional energy that was called "Minus Energy." One of these monsters was an Evil Twin of Ultra Seven.
The Shadow, the Source and the Hollow from Charmed. Cole became host to each power once.
The Shadows from Wraith The Oblivion. Each wraith has a nihilistic voice in the back of their head, urging them away from the things that defined them in life and towards oblivion. Wraiths who give in to this voice too deeply become Spectres, agents of Oblivion focused on destroying all existence.
However, the reverse is also true: Spectres have a Psyche, the voice of life and hope that tries to talk them into doing 'the right thing'.
Werewolf: The Apocalypse also had Bane Spirits, spirits of violence, anger, misery, and corruption used to harrow humans and turn them into fomori and servants for the Wyrm. While the Bane Spirits are born of mankind, the gameline makes it clear that just shoving an anger spirit in someone doesn't make them the Hulk; they need to give into its subtle insinuations first...
Fan-made expansion Genius The Transgression has an interesting variant in Manes, who represent popular concepts disproved by science. Entire worlds can be created this way — the moment the Viking probe sent back pictures of Mars as a barren wasteland, the Martian Empire came into existence. They're not always bad though, they just want to continue existing.
Additionally, while they're more like normal people than most, the Clockstoppers are anti-Geniuses; they possess no spark of creativity and reduce wonder and Wonders to emptiness. Did I mention the part where in place of a creative "spark" they possess a gnawing "void" that essentially means they run on spite?
In GURPS Fantasy II, the various monsters infesting the Mad Lands not only are often humans corrupted by their dark sides, but their naming even follows the "-less" format. E.g. people who don't speak out when they should become Soundless, people who are too quick to take offense become Skinless, etc.
The four gods of Chaos in Warhammer 40,000 are born from base emotions. Khorne is the God of Hate, Rage, Blood, War, Honor and Killing. Tzeentch is the God of Change, Hope, Ambition, Manipulation, Scheming and Sorcery. Nurgle is the God of Decay, Despair, Love, Destruction, and Disease. Slaanesh is the God of Decadence, Excess, Pain, Pleasure and Self-Indulgence.
The Circles of Hell in Obsidian.
In Dungeons & Dragons, there is the Fihyr (from the Monster Manual II.) To quote the description, "Fihyr are the collected fears of humanity made corporeal."
Demons in Pathfinder are born from the sins of evil souls, and it's mentioned that even a single such soul can spawn hundreds if not thousands of demons.
The Nightmares from Don't Rest Your Head are things from unfeeling bureaucracy to sleazy journalism to hatred itself given flesh, either by a person transforming into one due to severe insanity, or by more exotic means — the Ladies in Hating are either trained in the High School or created from young girls poisoned by the blood of other Ladies in Hating, while legend has it that Mother When came from... somewhere else when someone tried to pick the lock on a door out of the Mad City during the Thirteenth Hour. They are almost universally cruel, twisted, and vicious, incapable of mercy, compassion, or kindness... and the very fact that the Wax King doesn't match this description is sometimes taken as proof that he isn't actually a Nightmare at all.
Most Deadlands monsters are of this variety. The setting runs on Clap Your Hands If YouBelieve Fear, and the Big Bads, who are the primary Heartless of this verse, harvest human fear and use it to animate anything the local populace fears, be it jackalopes, wendigos or living blast shadows. These created abominations are powered by fear too, so they are also The Heartless.
The Horrors of Earthdawn and Shadowrun are Eldritch Abominations from another dimension that feed on pain and negative emotions. They vary wildly in their intelligence and habits, from near-mindless beasts that slaughter indiscriminately to diabolical masterminds who start wars and plagues. Then there are those like Yserthgrathe and Chantral's Horror, who go for quality over quantity - find a single "perfect" victim and dedicate your existence to inflicting misery on them, including making them immortal so that there is never any escape.
Magic: The Gathering gives this sort of origin to devils (as having come from demons) on the plane of Innistrad, as described in the flavor text of Riot Devils—"Devils are demons' unearthly desires made flesh."
Demons, angels and devils are all created from mana (aka the magic-powering energy directly tied to your philosophy and emotions), so potentially any of them can be born of negative emotions, provided there's enough mana.
Dark Force from Phantasy Star is a variation of this. While the ultimate villain of the series, the Profound Darkness, is locked in another dimension, its hatred is strong enough to break through the seal and take physical form, manifesting as the series's reoccuring nemesis.
The spawn of Dark Gaia in Sonic Unleashed are, according the the in-game bestiary, negative emotions which have manifested physically using Dark Gaia's energy. In addition, they'll also possess people at night, completely turning their personalities arounds, usually for the worse.
Monster Rancher Evo has these, but it's only apparent on the overworld stand-ins, as during actual battle the enemy monsters looks the same as if you were controlling them.
The Fiends from Final Fantasy X are the souls of anyone who died without being purified, either in the lack of regret in their soul in death, or via ritual after.
The Aeons on the other hand are the manifestation of the souls in a more positive way. By technicality they still are pretty darn close to the monsters in concept because they chose to become an unsent. However, unlike the monsters, they willingly gave up their bodies to aid the summoners so they retain their humanity.
The fiends a particularly tragic example, as unlike most they are not forcibly converted. Rather, they're left in an And I Must Scream state until the combination of their latent emotions and the solitude drives them completely insane.
A good many monsters in Final Fantasy XII were also created by powerful negative emotions.
In Final Fantasy IV, Cecil must battle the very incarnation of his own sinful past at the top of Mount Ordeals in order to become a Paladin.
In Persona 2, Nyarlathotep is revealed to be the grand Anthropomorphic Personification of the negative emotions of humanity as a whole — fear, hatred, anger, and every other flaw of the species. He's the Evil Counterpart to Philemon, who is similarly an Anthropomorphic Personification of human strength and positive emotions... and it's further revealed that the entire series up to that point was little more than a pissing contest between the two to see which was truly the more powerful aspect of humanity. And you do actually get to punch Philemon for this.
At one point during the events of Innocent Sin, Nyarlathotep also uses his power to make rumors reality to bring out Shadow versions of your entire party.
The Shadows of Tartarus in Persona 3 are essentially this, being born from the collective desires of any human who ever wished for death. This eventually brings about the The End of the World as We Know It when a mentally unstable man opens the door for them to call forth an Eldritch Abomination-level Shadow like Nyarlathotep: Nyx, the manifestation of death. In the end, the hero gives up his soul, not to heal the heart of humanity, but to reseal Nyx away from the collected weight of their negative emotions.
In Raidou Kuzunoha vs. The Soulless Army, the Hiruko are what happens when The Heartless gets combined with Body Horror, resulting in a race of parasitic monsters bred from the despair, hate and fear of the residents of the Capital who at the slightest provocation will erupt in a Nightmare Fuel-laden scene, converting the hapless victim into a Red Cape, or outright bursting out of the body. Neither variant is particularly pleasant.
Shin Megami Tensei IV shows a twist on this: the citizens of the Eastern Kingdom of Mikado turn into demons merely by reading the Literature the Black Samurai freely gives away. Turns out that for Casualries, who have been deprived of certain ideas (the most basic of socialist principles is treated as dangerous sedition), the realization that the system is heavily dependent on the Casualries being Gullible Lemmings drives them to demonic insanity.
The Noise in The World Ends with You follows this trope to a tee. Not only this but they will be drawn to negative emotions and can covertly possess people so that they appear to be "in a rut."
This is understandable, since this game was made by the same people who made Kingdom Hearts, and the Noise resemble Heartless with tattoos.
Matias in Tales of Innocence formed from Asras' collected pain and anger when he was stabbed by his own lover with his BFS.
Zerom in Tales of Hearts are creatures that feed on life force, causing a wasting disease called Despir Sickness that not only puts the victim into depression, but eventually leaves it a petrified husk. They are apparently drawn to people who are in the grip of negative emotions like fear, doubt, and grief, because people who have had such emotions amplified by MacGuffins invariably come down with a bad case of the disease.
Darth Nihilus from KotOR 2 became a living entity of pure hunger when The Dark Side consumed him. His most infamous power is a life drain that can instantly cause the extinction of a planet.
Dark Brain of The Great Battle and later Super Robot Wars, who feeds on negative energy and gets more powerful. He CAN get punched out by the heroes, but they can't kill him, since there's no way they can destroy the negative energies of humanity.
To that extension, Destiny's evil group Ruina also feeds on negative energies to make them powerful, doubled that they reside in a Crapsack World where lots of such things exist.
It's suggested several times that the power infusing DarkChips in Mega Man Battle Network 4 and 5 comes from people's hatred. (It also seems to come from a dark dimension called Murkland, but the two aren't necessarily contradictory.) As usual in these dark/light games, the bad guys are always ranting at you about how everyone is evil deep down, which makes it very satisfying when Mega Man smacks down Duo in the ending of 4:
Mega Man: Living creatures all have both good and evil in their heart. No duh!
The original-series antecedent of the above storyline is Mega Man 8's "Evil Energy". Rather than coming from dark thoughts, this stuff feeds on them — and it finds Wily an excellent source.
The Pokémon Shuppet feeds on the negative emotions of people.
Pokedex: "Shuppet grows by feeding on dark emotions, such as vengefulness and envy, in the hearts of people. It roams through cities in search of grudges that taint people."
In the Pokémon anime, this seems to be a good thing. When a shuppet feeds off someone's dark emotions, they feel a lot better. The Shuppet also means no harm.
While not a true creature, the Bittercold from Pokémon Mystery Dungeon: Gates to Infinity is a corruptive force in the form of an immense snowflake. It's created from the hopelessness, despair, and bitterness of the Pokemon world that is powerful to cause its end. It's so strong that anything near it is so overwhelmed with its power that it suffocates from the negativity. Even the player character, who is supposed to be immune to it!
Bogmire in Luigi's Mansion. More precisely, as the quote says, it's a manifestation of the fear and despair of the mansion itself: A product of the mansion's fear and despair. He's not sure who to fear or what to despair these days.
One of the bosses in Hype The Time Quest is the titular character's dark side, although it is never elaborated upon, and as such may not satisfy the trope fully.
Since Psychonauts takes place largely inside people's minds—-in particular, the patients of a mental hospital—-most enemies in the Mental Worlds are this in some form. Perhaps most explicit are the Nightmares, demonic-looking beasts that appear in Boyd's mind and locked up inside Milla's.
The Censors are, in some ways, the opposite of this—-they're essentially antibodies of the mind that destroy thoughts that don't belong, such as forms of insanity. Unfortunately, since you don't belong in other people's minds, they consider you fair game.
The Lords of Shadow from Castlevania: Lords of Shadow also count. When the three founders of the Brotherhood of Light ascended to Heaven as beings of pure good, the evil they abandoned possessed their mortal bodies and became the Lords of Shadow responsible for so much misery in the world.
The "blot" versions of eaten residents of wasteland in the final level of "Epic Mickey"
The Ixupi from Shivers are ancient Mayincatec demons that suck away the life essence of any nearby human beings.
The Negativitron of LittleBigPlanet 2 was born of the negative personality aspects of the creators of Craftworld. This is technically a spoiler, but the name alone is kind of a giveaway, and it's foreshadowed along the way besides.
In Ultima IX, the Guardian, dimension conquering threat from the previous several games, is retconned into the Avatar's Enemy Without: specifically, the embodiment of all the evil and darkness he cast off when he became the Avatar.
The Sha in World of Warcraft: Mists of Pandaria are born from the negative emotions of the people in Pandaria. Each Sha is aligned with a specific negative emotion, and their size is a direct measure of how strong that emotion is among the populace of Pandaria. At least six Shas are known at present: Anger, Despair, Doubt, Fear, Hatred, and Violence. The seventh is Pride.
Demons in Albion are physical beings intentionally created out of emotions of fear for use as living weapons.
Fate/stay night has Angra Mainyu, which was apparently created from the soul of a man who was made to suffer the burden of all of the evils of humanity. This soul later came into contact with a wish-granting artifact (the Holy Grail) which resulted in the soul graduating to full blown Eldritch Abomination, becoming an embodiment of all mankind's evil in fact rather than figuratively.
Kid Icarus: Uprising gives us the Underworld Army who are an extreme case Chapter 22 reveals that Hades uses the souls of fallen soldiers to create Underworld monsters, and thousands of souls are used to create one monster, thus wasting them entirely.
The Yamiko in the Sailor Nothing novel are a particularly gruesome version, with the interesting twist that their being pure evil also makes them incompetent, with no self-control or self-discipline (see the Quotes page for the full version of the page quote). This may be considered a deconstruction of sorts.
The Yamiko are also interesting in that they're made by magically cloning a human, with the cloning process doing nothing more harmful than knocking out the human. The first act of a newborn Yamiko is usually to murder the original, but not only is it possible for the whole and unaltered human to come face to face with the living embodiment of their dark side, it happens a few times over the course of the story, and if a Yamiko has let their human counterpart live, it's usually so they can be Forced to Watch as their Yamiko copy brings their whole world crashing down in the most sadistic manner possible.
Unicron's portrayal in Transformers Armada claims that he feeds on hatred, violence, et cetera. This is inconsistent with many other portrayals of Unicron, however, as usually he feeds on planets.
It's a bit more complicated than that. He eats planets, but the energy from hatred and violence is what gives him the strength to metaphorically get out of bed in the morning. The planet eating is mostly because all life offends him and he wants it gone. This is part of his upgrade from Giant Space Flea from Nowhere in G1 (eventually revealed to have been whipped up by some Mad Scientist on a whim) to God of Evil in the modern TF franchise. He'll exist As Long as There Is Evil (well, so long as there's hate), and his goal is the complete eradication of all universes in The Multiverse, resulting in a Void so empty that even space itself as we think of it no longer exists. He's done this to entire universes before.
Penelope Spectra from Danny Phantom was a ghost posing as a school guidance counselor. She made her patients more depressed, however, because she needed their negative emotions to keep herself young. At least, until she was able to make herself a new ectoplasmic body that wasn't falling apart.