Wendy: You're telling me that you — Pip — a priest — are willing to risk your life for me. Mirror Pip: We're all children of the Lord. Wendy: I'm in the mirror universe. ... A parallel universe where everyone who's good is evil, and evil is good. It's like that episode of Star Trek where Spock had a goatee and Chekov tried to — Mirror Pip: Star Trek. You mean the sci-fi series from The Sixties starring the great George Takei? Wendy: It is an evil universe.
Tengen Toppa Gurren Lagann performs this in Parallel Works 4, where the Beastmen are being oppressed by Kamina, and Viral is The Hero, it actually makes Kamina look legitimately evil.
Happened in one episode in Doraemon when Doraemon and Nobita enter and explore a mirror universe and interact with their counterparts. This universe is literally mirrored of having mirrored writing and east is west. Other differences include reversed gender roles showing Nobita's mom as the breadwinner, all males dressed as females, and different teachings in school. Nobita and his counterpart decide to swap roles for a day.
The movie Road to Ninja from Naruto Shippuden is based on this concept. Naruto and Sakura are transported to a parallel world where Sakura's parents sacrificed themselves for Konoha instead of Naruto and most characters exhibit the reverse of their normal personalities. Ex. Ino becomes a prude, Shino hates bugs, etc.
An episode of Pokémon has Ash travel into one when his Mirror Counterpart grabbed his Pikachu. The mirror counterparts of the main cast all have opposite characteristics: Ash is timid, and Pikachu is always attacking Team Rocket, who are celebrated heroes, Clemont is athletic, Bonnie is mature and ladylike, and Serena is aggresive.
There is a set of Magic: The Gathering involving a plane that transforms back and forth between its mirror opposites. Lorwyn is a bright, cheery world of eternal summer and daylight, filled with the stuff of whimsical fairytales. Then the world is abruptly transformed into Shadowmoor, stuck in perpetual twilight, and filled with the stuff of the Grimm brothers. Most inhabitants change with it, believing that they've always lived in whichever world it is (which could bring with it all kinds of metaphysical uncertainty about just how often the world changes its nature).
Also, Time Spiral block revealed several, including a consistent one in which Mirri instead of Crovax became the vampire evincar of Rath. Also the only male angel in Magic history.
Comics do this all the time. The DCU has its "anti-matter" Earth, wherein Ultraman, Superwoman, Johnny Quick, Power Ring, and Owlman (the Crime Syndicate of Amerika) are the evil duplicates of Superman, Wonder Woman, the Flash, Green Lantern, and Batman (the Justice League of America), and Lex Luthor was the only superhero left in the world. (A later story introduced the Justice Underground, a team of heroes led by Riddler's counterpart, the Quizmaster. And following Riddler's Heel-Face Turn, Quizmaster had a temporary Face-Heel Turn.)
The trope originated in the Silver Age with Earth-3, which was destroyed in Crisis on Infinite Earths but was restored later. The antimatter universe was based on this concept.
The animated Justice League also had the Justice Lords universe - a variation on the regular DCAU in which the death of The Flash resulted in the League/Lords becoming a totalitarian dictatorship.
A similar universe appears in Injustice: Gods Among Us, where Superman became evil and totalitarian after The Joker tricked him into killing Lois Lane.
Exiles, of the Marvel Universe, had an issue where Galactus restored worlds instead of eating them, and the Silver Surfer was a power-hungry despot who had destroyed his own homeworld. The first issue of Exiles was in a universe where Magneto was the benevolent teacher who wanted mutants and humans to live in harmony, and Professor X believed there could only be peace when humanity was eradicated.
The Age of Apocalypse has elements of this trope; Magneto leads La Résistance - including a Good Is Not Nice version of Sabretooth and a version of Colossus who is a psychotic Anti-Hero - and there is a human resistance including a heroic Dr. Doom and Sentinels actually protecting mankind, versus a dystopic mutant tyranny whose ranks include Cyclops and his brother Havok as well as Beast, now known as Dark Beast, as well as a new character called Abyss who was later introduced into the mainstream comics as a heroic character. However, though Dark Beast is a cruel and sadistic Mad Scientist and a despicable piece of work, Cyclops is just an Anti-Villain who does a Heel-Face Turn, and as the title suggests this is a world where recurring X-Men villain Apocalypse has achieved world domination and if anything has actually gotten worse; other villains like Mister Sinister merely serve as his minions and are still evil, while most heroes and villains from other parts of the Marvel Universe didn't change sides, they were just killed when Apocalypse nuked the United States and started purging humanity.
Played with in the Marvel Zombies universe; the heroes are all evil, but only because they succumbed to a Zombie Apocalypse and proceeded devoured most of the human race, though they are sentient zombies who take sadistic pleaure in ripping their meals apart. Magneto and Doctor Doom show up as their enemies and help rescue survivors, but mostly in the context of them being Noble Demon types who object to rampaging malevolent zombies taking over the planet. The cause of mutant supremacy doesn't matter much when most mutants and non-mutants have made peace with each other and decided just to eat everyone else.
The Thanos Imperative features the Cancerverse, a universe where Death has been wiped out by four powerful demons called the Many-Angled Ones, including Doctor Strange villain Shuma-Gorath (like the other three, a unique multiversal entity — it has no good twin anywhere in the multiverse). Life has become pollution and is spiraling out of control, and every living thing in the universe is Brainwashed and Crazy and are fanatically devoted to worshipping the evil entities, though non-living beings like the Vision (a robot) are immune and still heroic. Thus, in this universe hero teams like The Avengers are the servants of evil gods, though presumably so is every living villain as well.
Archie Comics' Sonic the Hedgehog comic has a Mirror Universe, called variously "The Reverse Universe", "Anti-Mobius", and "Moebius". In it Dr. Robotnik/Kintobor is a Kindly Vet, while the Freedom Fighters fight against freedom, having overthrown the previous monarch. The Anti-Freedom Fighters Suppression Squad all dress in black leather, and acted like a bunch of juvenile delinquents until Anti-Sonic became "Scourge" and conquered Moebius.
The interference from the heroes' world has caused the parallels to break down a bit; Scourge has been turned green, and hooked up with Fiona Fox for a while, Anti-Bunnie has pulled a Heel-Face Turn and wears Omega Armor instead of being a cyborg (anti-Rotor is the cyborg) and anti-Antoine briefly posed as his good counterpart. Basically the status quo for the mirror world gets altered more often than the main one. Oh, and Miles Prower even has the goatee.
Subversion: In Phil Foglio's short story "Work Ethic" found in Grimjack #40, heroes from a world in which there is only pure good and pure evil (and the heroes always win), get transported to Grimjack's world, which has a more realistically varied moral spectrum. Thus, since they see that everything is not purely good, they begin to destroy the entire town of Cynosure until Cynosure's protector sends them back to their own dimension. (Incidentally, these heroes, the Heterodyne Boys, later became the inspiration for Girl Genius.)
Similarly, in JLA-Avengers, the League, seeing the many imperfections of the Marvel Universe, conclude that it's the Evil Universe. The Avengers, seeing statues and museums to the heroes everywhere in the DC Universe, conclude the exact same thing (they think the DC heroes have set themselves as gods).
Quintessons are tree-hugging, peace-loving, groovy lingo speaking, space-hippies. The major Quintesson character is an exile who disagrees with the usual Quintesson policy of non-interaction with races until they're deemed sufficiently advanced — as opposed to the normal Quintesson policy of enslaving or arbitrarily executing "inferior" beings.
Earth is a Crapsack World that views both factions of Transformers as enemies. But more illustrative of the reversed nature of the universe are the Witwicky brothers, who are now petty criminal allies of the evil Autobots, and the machine-hating Marvel Comics villain Circuit Breaker (or Josie Beller), whose Shattered Glass counterpart is Josephine "Sephie" Beller, an ordinary human Decepticon ally who loves machines and admires the Transformers. The Mad Scientist Dr. Arkeville's counterpart is Professor Henri Arkeville, a Science Hero. The cruel dictator Abdul Fakkadi's counterpart is a benevolent ruler who is a father to his people. Cobra is a resistance unit fighting for freedom against America's tyrannical enforcers: G.I. Joe.
Nobody in this universe has heard of beings like Primus, Unicron, or The Fallen. When a displaced G1 Cliffjumper explains the concept to Megatron and the other Decepticons, they find the idea quaint. It should be noted that these beings are multiversal singularities (there is only one version of them in the multiverse), so if they ever showed up in the Shattered Glass-verse, they would be the same as in the normal universe. There is an evil Alpha Trion, however.
The Disney Comics also dabbled in this; in this comic, Paperinik ends up going into an alternate universe where Uncle Scrooge is poor, Gladstone Gander is unlucky, policemen are criminals, criminals are good, and Paperinik himself is evil.
Judge Dredd has Deathworld, homeworld of Judge Death and Dark Judges, where Judges realized that all crimes are made by living, so life itself was outlawed.
Mandrake The Magician has had several adventures involving a mirror universe. Usually the evil mirror-universe tries to invade the ordinary universe, coming through just about any mirror (though they prefer to begin with Narda's), since they have a spray that makes mirrors soft and permeable.
The Powerpuff Girls comic "Deja View" involved the Powerpuff Girls' adventures through a universe in which they must help the heroic Jomo Momo (Mirror Mojo Jojo) defeat Oppressor Plutonium (Mirror Utonium). Meanwhile, the girls' counterparts, the Powerpunk Girls, wreak havoc on Townsville. This was originally going to be a season five episode for the TV show, but its CGI put it over budget and near a tight deadline. They gave the storyline to DC Comics to make as issue #50.
The 7th arc of the My Little Pony: Friendship is Magic comic book series deals with the mane six (plus Spike) traveling to another dimension where the villain of the season 3 premiere, King Sombra, is actually The Good King while the counterparts of Princess Celestia and Princess Luna are the villains. Other changes include the mane six's counterparts being pictured on wanted posters while various antagonists throughout the series are depicted as heroes.
The RPG.net forum discussion Shattered Disc, a mirror universe Discworld. "The world burns ... on the back of a turtle."
A Bleach fanfiction features Sonoma and Kan'na. Sonoma is the original universe, introduced in the works of Tite Kubo. Kan'na is a universe created by a user, and is the parallel twin universe to Sonoma. Kan'na is war-torn, and the World of the Living is in a post-apocalyptic state, having been overrun by Hollows. Morals and standards do not exist in Kan'na, same for rules.
A group of The Transformers fans wrote several pieces about a possible Transformers mirror universe, with the first such piece appearing in 2001, predating the Shattered Glass universe by around seven years (as Shattered Glass #1 appeared in 2008). This fan-made Mirror-verse setting was unusual in that its good and evil flip was played completely straight (with the evil Autobots proving to be even more monstrous than the Decepticons of the normal universe), where Shattered Glass has hints of clever parody in its content.
Super Mario Bros. The Movie posits a "sub-dimension" created through the impact of the meteorite into earth that caused the extinction of the dinosaurs. The dinosaurs continued to evolve in this sub-dimension in the city of "Dinohattan", a city mirroring New York City chiefly through the city's design and the overall attitude of its citizens.
In Andre Norton's Star Gate (1958), the human colonists of Gorth, seeking an Alternate Universe version of their beloved adopted planet that has no native intelligent life, accidentally stumble into a version in which their own counterparts have used their advanced technology to enslave the inhabitants.
Characters from Mirror Universe settings occasionally turn up in the Nightside novels, such as Dark Artur (an alternate King Arthur whose mentor Merlin had sided with the Devil), or Joan Taylor and Steven Shooter (Gender Flip villainous versions of the series leads).
Spider Robinson's story "Mirror/rorriM Off the Wall" has an invasion by Trebor, the evil mirrortwin of Robert, a patron of Callahans Crosstime Saloon. One of the tipoff's is that Callahan's bar doesn't have a mirror normally, just quotes written behind the bar, and suddenly, there is a mirror.
The Mirage is a novel about a Mirror Universe War on Terror, where a liberated, cosmopolitan United Arab States is attacked by a fundamentalist Christian America and ends up invading them in retaliation. Of course, the fact that it's such a funhouse mirror version of our world is key to the plot...
In Doctor Who episode "Inferno", the Doctor was transported to a world where Britain was a military dictatorship and the UNIT characters were either evil (like Brigade Leader Lethbridge-Stewart and his Eyepatch of Power and Platoon Underleader Benton) or resignedly following orders (like Section Leader Liz Shaw and Doctor Petra Williams)note as the Star Trek example was not yet so dominant, the evil universe versions of both the Brig and Stahlman have less facial hair than the mainstream ones. The location and plot were the same (an attempt to drill into the Earth's mantle), but penetration was reached and the world was destroyed. The Doctor was able to escape in time and stop his Earth's version of the project.
Subverted in Stargate SG-1. A whole shipload of alternate SG-1 teams from various universes arrives. One team ends up hijacking the Prometheus. Mitchell says to his double, "You don't have beards, so I know you're not from the Evil Twin Universe". It turns out that this particular team comes from a universe in which Earth does not have a working Zero Point Energy module and needs one to power their defenses. So out of desperation they've contrived the conditions that caused the dimensional travel so they can steal someone else's. You'd think they'd just get all the Samanthas to work on the problem. Which is pretty much how they solve the problem of sending everyone back.
Charmed had a Polar Opposite World, where good and evil were reversed. The characters had to forge an alliance with their "evil" selves to get both worlds back into balance.
An imbalance that occurred when these universes crossed caused a total Flanderization of their respective moralities; in the 'good' universe, even the most minor of infringements of law or courtesy was enough to have you (cheerfully) shot, whereas so much as the slightest gesture of kindness in the 'evil' universe would incur the same consequence. Also, it was always day in the good world, and always night in the evil one.
Given the nature of Charmed, it also resulted in changes of what the characters were - the witches became warlocks, Whitelighters became Darklighters, and so forth.
The Star Trek Mirror Universe started out (in the Star Trek: The Original Series that introduced it) as identical to the main universe, except that for the moral inversion between the Federation characters and their evil Empire counterparts. When the Mirror Universe was revisited in Star Trek: Deep Space Nine, the correspondences were less straightforward — the Empire had been overthrown, humans were downtrodden slaves, and the station was run by Kira's evil counterpart.
And, despite being the Trope Codifier, slightly subverted it - even though his mirror counterpart had a Beard of Evil, Spock was, as the show put it, "A man of honor in both universes", and not strictly evil in the mirror universe despite clearly being on the side of the angels in the primary reality. Further, the Aliens Of The Week were Actual Pacifists in both the 'normal' universe and in the Mirror Universe.
When the prequel series Enterprise revisited it in "In A Mirror Darkly," it also completely changed its opening credits' entire mood from "Vapid Naive Hope" to "War! Conquest! Exploding Frickin' Laser Beams!" Moreover, since almost everyone except maybe the Vulcans is shown to be evil in one way or another, the ending in which The Bad Guy Wins doesn't really come as a surprise. What does come as something of a twist is it's the bad gal who wins, and then promptly proclaims herself Empress.
One of Shatner's novels gives a quite satisfying explanation for why this is the only parallel universe open to cross-dimensional travel. When the Enterprise chased the Borg back to Cochrane's time in First Contact they didn't, as they supposed, change the timeline and change it back again: they gave rise to a separate timeline. (Then returned to their own and assumed, since it looked the same, that they had succeeded.) Their clumsy attempts to cover up their fixing didn't fool the Vulcans, who deduced the existence of the Borg, concluded that the logical counter to this menace was the savage militarism they discovered on Earth, and set humans on the path to becoming an Evil Galactic Empire that would be big and bad enough to defeat the Borg when they appeared. Because of the artificial way it was created, this timeline remained entangled with its parent one, leading to a leaky boundary, but also to cross-influences producing similar people with the same names in the same places despite completely different backstories.
The Star Trek novels continue the Mirror Universe after season seven of Deep Space Nine, showing us how the Terran Rebellion is becoming more of a social revolution, as well as giving us perspectives on Mirror Spock's rise to power as Emperor after the initial episode "Mirror, Mirror", Empress Sato's putting down of the rebellion during the Enterprise era, and how Picard stopped being a slave and became a Terran Rebellion leader during the TNG era.
Also, the Expanded Universedoes show us more than one version of the Mirror Universe, though whether it's intentional or simply the Trek EU not being having nearly as tight continuity as the Star Wars one is unknown. However, one book features the ISS Enterprise-E of the Terran Empire, which doesn't work with the DS9 incarnation of the MU, so that one is clearly alternate from the beginning.
It should be noted that the show does occasionally show us parallel universes that are not the 'Mirror Universe.' In the Star Trek: The Next Generation episode "Parallels", Worf is stuck traveling between different realities in which there are mostly only minor differences. The only semi-villainous alternate main character was a Riker from a timeline in which the Borg had assimilated much of the Federation, who fired on Worf's shuttle, desperate to avoid returning to his native universe. He does sport a much more wild and unkempt beard than our Riker, but it is clearly a Beard of Sorrow rather than a Beard of Evil.
The IDW comic story "Mirrored" gives us the Mirror Universe of the New Movie Universe. As Captain Spock vanquishes the Klingons in the name of the Terran Empire, Commander Kirk vists Rura Penthe to get revenge on Nero, (who in this universe never escaped, and has been there since the attack on the Kelvin). Then Kirk ends up with control of the Narada, and things get worse.
One episode of Voyager opened with what appeared to be a mirror universe setting, with a vicious Janeway threatening a planet to get resources. It turned out to be an inaccurate historical record written by this civilization some time in the future, based on an incident with Voyager several hundred years ago. The holographic doctor's backup unit is later activated and sets the record straight. Those familiar with Deep Space 9's episodes likely knew it wasn't the case, those who hadn't been watching...
In an episode of Power Rangers Ninja Storm, Tori is sent into a parallel universe where the other rangers are the bad guys and the villains are good guys (the shallow, fashioned obsessed villainesses are even hippies in this universe).
One of the many, many sphere malfunctions in Seven Days, rather than sending Parker into a Mirror Universe, actually inverted the real universe (Since the existence of parallel universes was disallowed by the show's Applied Phlebotinum), changing Never Never Land into the seat of a tyrannical dictatorship, Ramsey into a spaced-out hippie, and reversing all writing. Parker, being morally ambiguous to begin with, was immune. Also, everything in this "inverted universe" is mirrored. Get it?
The Middleman: In "The Palindrome Reversal Palindrome", the alternate-universe Middleman goes from all-American Boy Scout type to less-than-altruistic Anti-Hero, Pip goes from selfish brat to Catholic priest working to help the helpless, Lacey is a stripper, with Noser as her muscle, the entire world is a dystopia ruled by Fatboy Industries, and Wendy is the Big Bad. Oh, and all the male characters have beards.
Red Dwarf played the concept straight in the episode "Only the Good...", where Rimmer travelled to another dimension where everything was opposite to procure the antidote for a virus infecting the ship. The alternate Rimmer and Hollister's positions and personalities were switched, with Rimmer becoming the captain and Hollister a technician on probation from a prison sentence; the Cat was the ship's chief scientist, and Kochanski was his ditzy secretary. (Also seen in deleted scenes: Lister was the Officer and a Gentleman first officer with no dreadlocks and a moustache.) Additionally, all writing was reversed and Rimmer became left-handed and well-endowed.
The concept was also played with earlier in the series, though not quite played straight. The earliest example is the episode "Parallel Universe", which had a universe where everyone's gender was swapped. Except for the Cat, whose counterpart was a male humanoid Dog.
Also played with in "Dimension Jump", the episode which introduced Ace Rimmer. Ace himself is an obvious mirror version of Rimmer, being a confident heroic and sexy test pilot, as is Lister's counterpart "Spanners", who is a successful engineer who is married to Kochanski with twin boys. Some of the other characters played by the main cast also contrast in this way: the Cat's counterpart, the Padre, is selfless and friendly; Kryten's counterpart, Admiral "Bongo" Tranter, is a human in a position of authority.
Also also played with in "Angels and Demons", where an experiment gone awry cause Red Dwarf to split into three parts: the cast and crew as we know them, a perfect Red Dwarf where everyone's a pacifist, and a decrepit Red Dwarf where everyone's a sadist. Hilarity Ensues when the three parts interact.
Mystery Science Theater 3000 explicitly parodied the Star Trek mirror universe in their Last of the Wild Horses episode. Tom and Gypsy are swapped with their evil counterparts. In the evil universe, Dr. Forrester and TV's Frank are trapped on the SOL, while bearded Mike and the bots subject them to horrible movies.
Hercules: The Legendary Journeys had a universe where Herc was an evil ruler called the Sovereign, and Iolaus was his jester. Opposing the Sovereign's rule were such figures as Joxer (the Spock "one good man" type character, but "opposite" to "our" Joxer by being competent) and Ares, god of love.
The Sovereign also had a Beard of Evil. Unfortunately for him, he doesn't realize that the main universe's Ares is also evil and has no qualms about killing.
Kamen Rider Dragon Knight uses this concept literally - the parallel world of Ventara is connected to Earth through reflective surfaces like mirrors and windows. The Earth Kamen Riders are mirror twins of the original Ventaran Riders, who are all heroes, which causes some interesting moments. For example, the good guys are a bit weirded out by Ventaran!Strike, since his Earthly counterpart is a Smug SnakeJerkass. He ends up in a relationship with the Hot Scoop at the end of the series.
Kevin, Gene, Feldman and Fargas in The Bizarro Jerry episode of Seinfeld.
Not played straight, but homaged in Fringe, where practically the first person encountered in the alternate universe is played by Leonard Nimoy.
Lost in Space did an episode entitled "The Anti-Matter Man" where John Robinson gets kidnapped by his mirror self, who wants to take his place in the "normal" world. In the mirror world, everything is different, where because the robot is good, his mirror counterpart is permanently imprisoned. Oddly enough (since it was slightly earlier) Drun (the mirror Don West) has a beard. Will, Dr. Smith and the Robot rescue him, and the episode concludes with John throwing Mirror John over the side of the "bridge" between the normal world and the mirror world.
Parodied in Community. When the tossing of a die to choose who picks up some pizza creates seven alternate universes, one horrible one is created where Pierce dies, Jeff looses an arm, Shirley becomes an alcoholic, Annie suffers a mental breakdown and Troy's larynx is destroyed. Genre Savvy Abed deduces that they're in the "evil" universe, makes them all fake beards and makes it their job to break into the true universe and wreak havoc.
Polish sitcom Swiat wedlug Kiepskich has an episode where the son of the titular disfunctional family passes by the mirror in the hall and discovers that his reflection is wearing glasses, formal suit and rather nerdy haircut. After brief argument with the reflection, he finally responses with a headbutt which results in them both swapping places with each other and he ends up in an alternate universe where his family is rich, their alcoholic neighbours are tennis-playing scholars with academic degrees, and he's a world famous fiddle player. Who is about to have his biggest performance in just few hours. Hilarity Ensues.
Here's an example you might not expect: in a "The Word" segment on The Colbert Report from February 10, 2007, Stephen opens with reporting on a new theory about parallel universes. In the middle of the segment, there is suddenly a cheesy science fiction zoom out, and suddenly Stephen is a left-wing hippie whose "Word" was "Troops Out Now!" After a while of seeing Hippie!Stephen's reporting, the same zoom out is shown, and you cut to "real-life" Stephen arguing in character that the US military should invade any parallel universes we find.
Mythology and Religion
Some would say the myth of Thor in Outgard (Thor:OutgardLoki, Loki:Logi, Thjalfi:Hugi).
The fan-made Brighthammer 40k campaign setting for the Warhammer 40,000 roleplaying games, is largely a Mirror Universe, with some Bizarro Universe mixed in — given the state of the normal universe, it's a pretty nice place to live. There are exceptions, however. The Tau are exactly the same — their new status as villains is simply because the rest of the universe now looks better, rather than worse, by comparison, and the Tyranids are also the same — they're mirrored in that they're now the threat another extragalatic faction is fleeing from, rather than being implied to be fleeing from an even worse extragalatic faction. While the Eldar are reversed in alignment as well, the makers took it to a logical conclusion: The Bright (Dark) Eldar were born in dire straits, and are fighting a losing war against their cruel brethren. The Slaan are a borderline exception, as well: They're much the same, but they're now villains as the setting replaces the mysterious-but-benevolent Old Ones of 40k and Warhammer Fantasy with the twisted and horrifying Great Old Ones; being devoted servants to the Old Ones is now an unequivocally bad thing. The Deceiver (the worst of the extremely nasty C'Tan in Canon) is still completely evil, it's just that how he goes about it reversed — he's known as the Soothsayer, and rather than causing trouble with deception, he instead specializes in sharing dangerous and unpleasant truths and dispelling even harmless or necessary lies. The other C'Tan play this straight, though — the Daybringer is flat-out benevolent, the Void Dragon is well-meaning but utterly alien and unaware of just how dangerous his knowledge can be in the wrong hands, and the Outsider is in a self-imposed exile due to emotional and mental scars from battling the Great Old Ones... but the Tyranids are heading toward his "prison" and no-one knows how that will turn out.
Another version of this is the Dornian Heresy which remains Grim Dark but switches the places of the loyalists and traitors and in almost every major event the alternate choice was made. The Emperor sided with Magnus during the Council of Nikea, meaning that the Thousand Sons gained the power to banish and in some cases even outright destroy daemons. Angeron had Horus and the Emperor side with him during the slave uprising on his world, resulting in him removing the combat implants in his warriors. After the Heresy the Legions were never broken up to make sure they could stay strong and root out traitors within their ranks. Ultramar was tricked into fighting the Alpha Legion and Word Bearers resulting in them becoming a third power, meaning that they were no longer held by the rules of the Imperium and began to tinker with their technology. Lorgar chose to declare a holy war upon Chaos rather than siding with them. And, most shocking of all, Abbadon became a tactical genius!
City of Heroes has the "Praetorians", evil world-conquering versions of the "normal" world's main heroes. Of course, it's up to the player character(s) to defeat them and ensure that they don't extend their conquests to other worlds.
They have a Greek name, and Tyrant wears Greek-style armor, because of a legendary Real Life incident: when told of the existence of alternate universes, Alexander the Great wept: "So many worlds, and we have not yet conquered one."
Counterpart to Odysseus, leader of the Warriors, is a trainer in the Underground.
Counterpart to Calvin Scott, husband to Aurora Borealis, is the leader of the Resistance.
Counterpart to Venessa DeVore, leader of the Carnival of Shadows, is the leader of the Carnival of Light and is responsible for the Resistance's very existence. She saw the Emperor's new order coming and banded together several military leaders to stand against him. Her psychic abilities also keep Mother Mayhem and the Seers from finding the Resistance.
And the fans love to embrace this, creating Praetorian counterparts of their own characters, and of currently unseen canon characters.
The same game featured the "Amerika Korps", who were from an Alternate History where, you guessed it, Hitler conquered and occupied the US. Clumsily Ret Conned into the Council Empire later on, though Issue 15 brings back the most memorable character of the Amerika Korps.
The "Amerika Korps" could be seen as a slight subversion to the idea that they were identical mirror counterparts. While the game's background clearly stated that the Statesman and the Reichsman could spend all day slamming each other into the concrete, the Reichsman did not like competitors approaching his power level and deliberately made sure his "teammates'" training were less than adequate. That came back to bite him in the ass.
Shadow Mirror isn't a one-for-one Mirror Universe, though — while some characters are flipped (most prominently Kyosuke Nambu/Beowulf), others were mostly the same or didn't even last that long (Ryusei Date was apparently the same up until Beowulf killed him, and Rai Fujiwara was killed test-piloting the original Huckebein). In Advance, it's stated that Shadow Mirror's Amuro Ray died young, paralleling the Mobile Suit Gundam novel rather than the anime.
The AdventureQuest Worlds 2nd Birthday Event has your hero visiting the Mirror Realm, where pretty much everything is switched around. Battleon is called Battleoff, Aria owns a butcher shop instead of a pet shop, Yulgar's Inn is called "Ye Olde Outt," Zorbak is actually good, Cysero is actually sane, Warlic is a Mad Scientist rather than a mage, and the greatest hero of this reality turns out to be Drakath. His "Lords of Order" (basically good mirror counterparts of the Chaos Lords you've fought so far) have been captured by this realm's Big Bad, King Alteon, who commands an undead army led by Artix, who is an undead Doom Paladin in the Mirror Realm, and you have to free them. The adventures include a trip to Lightovia (basically a mirror Darkovia) where Safiria is the Queen of the Werewolves instead of the Vampires (and where the werewolves love silver rather than it being the bane of their existence), a trip to a farm where a farmer is infecting his chickens with Chaos (don't ask) and finally heading to Alteon's secret lair, an evil mirror version of Swordhaven Castle, to do battle with undead Artix.
The Legend of Zelda Oracle games basically take place within two different universes that are mirror universes of each other. Subverted when you play one game with the code you get from another, where some people from Labrynna are in Holodrum and vice versa, depending on which game you play second.
Many team vs. team combat games and multiplayer RTS/turn-based strategy games implicitly use this for the sake of fairness; both teams will have access to identical characters who fight in levels that are the same on each team's side. Color-coding or something else that doesn't affect gameplay distinguishes the two "worlds."
AH.com: The Series has the Mirror Crew, though we never see their home universe. They're stereotypical pure evil much like the Star Trek original, but the dynamic is quite different because the primary AH.com crew are scarcely angels themselves, it's more a case of an "upgrade universe".
The RPG Net message boards once featured a wonderful idea for a Discworld mirror universe. The Disc is torn by war between the scarily charismatic Last King, ruler of Ankh-Morpork and the Plains, and the all-powerful Crone, who controls the Ramtops. Between them are the Crone's former apprentice Magrat, Havelock the assassin and Samuel King-killer. And their leader, the last survivor of the Silver Horde, Rincewind Spellholder.
Later additions included the Wizard-Killer, a bestial creature that haunts the library of the abandoned UU; Susan the Vain, who plays both sides, seeking to replace her grandfather as Champion of the Auditors; the History Monks, who presumably have decided this is all meant to happen for reasons of their own; the Chalkland Hag, who seeks to challenge the Crone and has turned a society of harmless brownies into an unstoppable army; Lady Sybil, who breeds war-dragons for the Last King and seduces others to her own ends; and Reginald Shoe, who has fled Ankh and turned Pseudopolis into an undead police state. Amongst others.
Gaia Online recently featured a literal Mirror Universe in the form of the Dark Reflection random item generator. Unlucky Everygirl Kanoko get sucked into a bizzaro Gaia, and Gaians have to venture through the mirror to rescue her. Bizzaro Gaia features strange versions of the most famous Gaian NPCs. Some of these are fairly basic changes. (Agatha and Rina swapping ages, Moria and Sasha swapping clothing styles, Ian and Rufus swapping species), while others are a bit more bizarre. (Scheming con man Nicolae is now a priest, Liam has become a woman, his yaoi-bait roommate Gino has become his boyfriend, and Edmund has become a huge nerd.). Various inversions of items are present as well. (Gaia-Tan has become Gaia-Sama, Grunny has become Prunny, etc...)
Mr Welch parodied the concept with a superhero player character that was, as usual, rejected by his gamemaster.
325. Even if he was a paragon of humanity in his alternate dimension, Good Hitler is not an appropriate superhero concept.
In Welcome to Night Vale, Desert Bluffs is implied to be a Mirror Universe. A sandstorm rolls into Night Vale, bringing with it doubles of all the citizens, who fight each other to the death. It opens a portal in Cecil's radio booth, transporting him away to places unknown, and depositing another man in his place. In the episode's P.O.V. Sequel, we find out where Cecil went, and learn that Night Vale isn't actually all that bad, bycomparison.
In this strip of Dinosaur Comics, where every comic is the same six images every time, an early story arc involves a mirror universe that is the same six panels... mirrored. Also, every character has goatees.
Following the history reboot, there have been twoforays to the evil mirror universe. Thus far, the only visible differences are the beards, the arrangement of the panels, and Dr Jones Sr.'s dislike of sausages and avoidence of puns.
In Wilys Defense takes place in a universe where Dr. Wily would like a less exciting lifestyle, Cut Man's the hero, Dr. Light is an egotistical megalomaniac, and X is Ax-Crazy.
Sluggy Freelance features an infinity of alternative dimensions. One of them is hinted to be a world of evil counterparts, having spawned a malicious version of Kiki, and possibly a selfish jerk version of Torg. (It's not known whether those characters were actually from the same dimension, but they might have been.)
The heroes of the Global Guardians PBEM Universe have encountered two of these. The first is the standard version where the various characters are morally aligned opposite to their normal nature. In this one, the heroic Global Guardians have their counterparts in the villainous Global Gladiators , a criminal syndicate that has taken over the world. The second was a world in which the Global Guardians had taken control of the world "for the betterment of mankind" after a much more severe Xorn invasion had left humanity on the brink of extinction.
While Megaman and Bass swapped intelligence with Protoman (They're geniuses and Protoman's the dumbass), Bob and George swapped temperaments: George became a blind psychopathic murderer, Bob became a flaming homosexual. The regular Bob was...a bit freaked out.
The Dimension of Hackneyed Stereotyped Opposites in Casey and Andy. Since Casey and Andy are Chaotic Neutral at best, conventional moral flips don't apply to them (instead Andi is a girl who dates God instead of Satan), but Quantum Cop's counterpart is Quantum Crook.
El Goonish Shive, obviously. There's the Main Universe's Tedd (the one the story follows), Beta Tedd, Lord Tedd (also known as Alpha Tedd and the evil one pictured in the mirror in the trope image note the image is not from EGS itself but from a Wotch special, the Tedd from Ellen's "Second life" dreams and those are just the canon ones. There's probably more we haven't been introduced to.
In Universal Compass there are 10 Alternate/parallel worlds that each reflect a certain emotion.
In The Non-Adventures of Wonderella, Wonderella and Wonderita end up in one of these...except Wonderella's morality is so fluid in the first place (due mainly to her It's All About Me mindset) that her mirror counterpart is virtually the same person as her.
One Rugrats has Chuckie and Tommy thinking that they landed in "Mirror Land" when they flipped over a mirror. Hilarity Ensues.
South Park parodied the Star Trek episode with their own Mirror Universe, from which visited an alternate Cartman. Exactly like the alternate Spock in "Mirror, Mirror", the alternate Cartman was bearded — but being the moral opposite of the "real" Cartman, he was of course kind, soft-spoken, polite and gentle.
Darkwing Duck has the Negaverse from which one version of Darkwing Duck's Evil Twin Negaduck originates, where he evidently rules with an iron fist. Nega-Honker has the (regular) Tank's personality and morality while Nega-Tank has Honker's, the rest of the Nega-Muddlefoots and Nega-Launchpad are thuggish psychotic killers, and the Negaverse versions of the other four members of Negaduck's "Fearsome Five" are known as the "Friendly Four" until Darkwing Duck retrains them. Interestingly, despite being Negaduck's ward, Nega-Gosalyn has the same general morality as her prime universe counterpart (much like bearded Spock), but a reversed personality: whereas regular Gosalyn is a rough-and-tumble sports-loving Tom Boy and Bratty Half-Pint, Nega-Gosalyn is a sweet and obedient well-behaved pink-dress-wearing young lady.
In Batman: The Brave and the Bold, Batman travels to one of these when the Red Hood, an alternate version of the Joker, needs his assistance. He seems to enjoy repeatedly punching out the alternate version of Green Arrow a little too much...
Justice League: Crisis on Two Earths depiction of Batman's counterpart Owlman is far more representative of an 'evil version' than the Justice Lords Batman. Owlman sums up the entire topic of alternate versions nicely to Superwoman (evil version of Mary Marvel, counterpart to Wonder Woman):
Owlman: Every decision we make is meaningless because somewhere, on a parallel Earth, we have already made the opposite choice. We're nothing. Less than nothing.
Superwoman: How can you say that? We're rich. We're conquerors.
Owlman: [pointing at alternate Earths] And here we're poor. We're slaves. And here, our parents never met, so we were never born. Here, the world ended in nuclear war. Here, no fish was brave enough to crawl up on land and humans never evolved. And so on, ad infinitum.
And summed it up even more succinctly to Batman later on:
Arthur: They look familiar. Did we play soccer against those guys?
Buster: No way! I'd remember a bunch of goofy-looking kids like that.
An episode of Recess had the same idea, when Principal Prickley makes them compete against a kickball team from the school where his brother is the principal. There is a nerdy kid like Gretchen, a typical-loser kid like Gus, a super-athletic kid like Vince, etc. This means that they are perfectly matched and it seems like one team will never beat the other. However, instead of one side being clearly "good" and the other "evil" the kids all conclude at the end that both of the principals are nuts and go off to play on the jungle gym, "Big Crusty" (which looks exactly like "Old Rusty" from the other school).
The 6th-season The Simpsons episode "Lemon of Troy" did the same swap, between two towns (Springfield and Shelbyville) instead of two schools. Bart and his friends all had Shelbyville counterparts, although they weren't so much evil (since Bart is not exactly good) as just antagonistic to our heroes. Milhouse and his counterpart even made friends.
Milhouse2: But Milhouse is my name!
Milhouse: But I thought I was the only one!
The SWAT Kats episode "The Dark Side of the SWAT Kats" featured the titular team being warped to a dimension where their equivalents are evil (as is that of their ally, Deputy Mayor Callie Briggs). The universe wasn't entirely swapped however - some major characters retained their 'correct' moral alignments. (There were other more subtle changes as well, such as the Enforcers using fixed-wing aircraft rather than helicopters.)
Megas XLR has a version of this in its two-part "Rear View, Mirror Mirror" storyline. In this timeline, main character Coop abandoned the titular Megas shortly defeating the series' Big Bad - losing his mind as boredom and battlelust sank in, culminating in the conquest of earth and several solar systems. Coop seems more offended at his alternate self being athletic and muscular (as opposed to...large) rather than evil, though.
The World's Greatest Superfriends had such an episode, "Universe of Evil". Each of the Superfriends received an Evil Makeover (Aquaman with an eyepatch! Robin with a pencil mustache! Wonder Woman letting herself go!) See them here.
Buzz Lightyear of Star Command had as a minor villain the imaginatively dubbed and yes, goateed, Evil Buzz Lightyear who came from a parallel universe that he had conquered. He's exactly as evil as Buzz is good. Also, there are parallel versions of the other main characters, except none of them are evil.
Codename: Kids Next Door had one in which even the acronyms were reversed - the counterparts to the Delightful Children From Down The Lane were the Little Traitorous Dudes From Children's Defence, who opposed the iron grip of the Destructively Nefarious Kids, who were led by Numbuh -4, who had attained a goatee to make himself look even eviller. Fortunately, since the regular Numbuh 4 is brave to the point of recklessness and a strong physical fighter, Numbuh -4 was a coward and weakling. Also, Numbuh -86 was a sweet girly-girl.
Other differences include a dumb and ill-tempered Numbah -1, Numbah -2 was also dumb but good at making jokes, Numbah -3 is a bitch, and Numbah -5 is a geek. Strangely, the LTDFCD still talk in unison like their main counterparts, though in surfie accents.
G.I. Joe's two-part episode "Worlds Without End" had several of the Joes traveling to a Crapsack World that bordered on this; while most of the alternate versions of G.I. Joe and Cobra were still morally similar to their counterparts in the regular universe, Cobra's having conquered the world made it the legitimate government of that world and G.I. Joe an enemy of the state. Also, in a bit of a twist, the alternate Baronness was romantically involved with Steeler's counterpart before he was killed in action, and this was one of the stronger influences which persuaded Steeler to stay with her in her world at the end when most of his teammates returned to their own world. (A couple of the others also stayed with him to help revive G.I. Joe's resistance movement in that world.)
Hey Arnold! had an episode where Arnold goes into the country to visit his weird cousin Arnie. All of the people he meets are counterparts of his friends, but they all appear in pairs, with each one displaying the attributes of the other's counterpart: Harold and Rhonda appear as Harry and Rhoda, with Harry being slim and fashion-conscious and Rhoda being a fat slob. Stinky and Phoebe are seen as Stumpy and Fifi, with Stumpy being intelligent and Fifi being simple-minded and Gerald and Sid appear as Gerard and Kid, with Gerard being the whiny one and Kid being cool. In the end, it turns out that it was All Just a Dream.
Sonic Underground had the hedgehogs enter one where they were the tyrants and Robotnik was a Freedom Fighter. It ended with them redeeming their evil counterparts. Strangely, there was no mirror version of Queen Aleena.
Ben 10: Omniverse: Ben 23 has a take on it, where the Ben there is a Jerkass, and he fights a bunch of Bad guys, including a Tetrax with a diamond Beard of Evil. In the end, it turns out the Big Bad was really Azmuth, trying to get back the Omnitrix from Ben who was abusing it, and Ben himself wasn't evil, just misguided. Since Tetrax was a good guy as well, the only real alliance difference was Seven-Seven who worked for Azmuth and thus was also good.
BIONICLE has two. The "Melding Universe" is a world where the Great Beings managed to fix Spherus Magna before it blew up, Toa look like Matoran (and vice versa), and the regular universe's Big Bads (the Makuta) embraced light rather than darkness. The "Dark Mirror Universe" is a world where the Toa became Knights Templar and conquered the world. The Makuta, the Dark Hunters, and a rag-tag group of renegade Toa form the resistence.