Mirrors are inherently creepy objects. If you're looking in a mirror, there's also someone looking at you. A movement in a mirror can make you jump. It's no surprise that they're associated with badness.
But what if there were something more than just one's own reflection in the mirror? What if the mirror showed something else looking back at you?
This is the Mirror Monster! Though a mirror is really just a pane of glass and silver, in fiction it can become a gateway - something terrible could come out... or something terrible could try and suck you in. A Mirror Monster could be an image only visible in a mirror. It could be a ghost fettered to the world of the living by a mirror. It could be something which comes out of a mirror or communicates via mirror. The key thing about this trope is the horror is directly connected to the mirror itself.
Sometimes mirror monsters can be defeated by smashing the glass. Unfortunately, sometimes smashing the glass can free the mirror monster and allow it free rein (or make even more of them).
Compare with Mirror Scare, Mirror Routine.
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Anime and Manga
JoJo's Bizarre Adventure has a couple of Stands that are mirror monsters. The first one (J. Geil's "Hanged Man") attacks your reflection in the mirror (or any shiny surface), which in turn damages you. The second (Illuso's "Man In The Mirror") actually pulls you through the mirror into the "mirror world".
In Kazuo Umezu's Scary Book: Reflections, the vanity of a girl who admires her beauty incessantly in a mirror causes her evil reflection to escape and attempt to take her place, driving her to the edge of insanity in the process.
This story was parodied in the Ranma ˝ manga, where the mirror is haunted by the girl's lingering presence, and now whoever looks into it will have their reflection pop out. They aren't so much evil as just incredibly annoying.
In one SlayersNon-Serial Movie, Lina and Naga ran afoul of a mirror that created clones of them. This mirror, showing the exact metaphysical opposite of the person who looks into it, creates a wimpy, demure Naga and a pathetic, weepy Lina.
Sailor Moon used this trope all throughout the Dead Moon arc of SuperS/early Stars, where Queen Nehelenia would trap people in mirrors in a "nightmare", a mirror leads to a portal to her evil dreamland, and in one episode she creates a house of mirrors and controls the Senshi's reflections and makes them psychoanalyze and hypnotize them, to convince them to give up.
Pokémon: Giratina and the Sky Warrior: Giratina travels through mirrors to get to its home dimension, the Reverse World. When it is trying to capture Ash and friends, it uses reflective objects to open a portal through and drag them via whirlwinds into the Reverse World.
There was a cartoon by Charles Addams (famous for inventing the Addams Family) in which a man in a barber chair with mirrors on either side has a virtually infinite series of reflections, one of them about three-fourths of the way down the line being of a monster sitting where he is sitting.
This is the modus operandi of The Flash rogue Mirror Master.
Legend: Darkness first appears to Lily by stepping out of a mirror.
Prince of Darkness: Satan's father (the "Anti-God") is on the other side of a mirror trying to get through into our world.
Phantasm: At the end of the movie, the Tall Man emerges from a mirror to grab Mike.
The movie The Broken deals with mirror monsters taking over their originals' lives.
Near the end of A Nightmare on Elm Street 3: Dream Warriors, dream invading serial killer Freddy Krueger leads the remaining kids to a room full of mirrors. They see him standing in the largest mirror, upon which he duplicates himself into every mirror in the room and drags them inside kicking and screaming. Fortunately, Joey learns to use his powerful voice — his own dream skill — to attack Freddy and free all his friends.
In Hellraiser: Bloodline, the Cenobite Angelique kills one of the future soldiers by appearing in a mirror in her human form as a woman pleading for help before dragging him in and decapitating him.
In The Shadow of the Wind, Julian, as a lonely child, made up stories that he told to other kids that he had a sister who came to visit him through the mirror and lived with the devil.
In the fourth installment of ''Odd Thomas'# by Dean Koontz, something briefly materializes out of a mirror to collect someone's soul.
In the Discworld novel Witches Abroad, the villain uses mirror magic, and when she loses control at the end, her reflection reaches out of a mirror and pulls her in.
The Dresden Files: This is the reason Harry refuses to keep mirrors around his house. A lot of nasty critters can use them as doorways between the Nevernever and the real world, like the fetches in Proven Guilty.
Played with in John Harding's Florence and Giles. Florence, the twelve-year-old narrator, discovers that Miss Taylor, her new and possibly undead governess, magically inhabits all the manor's mirrors. Then again, Florence is both an Unreliable Narrator and completely Ax-Crazy.
The Polish book Bromba i inni (featuring descriptions of numerous fictional creatures) includes the "Wild Viceverses", mirror-dwelling creatures which are created whenever two mirrors are placed opposite each other. While they look more or less like warped abominations, they're a subversion in that they're rather amiable and benevolent. That said, they love playing tricks on vain people by suddenly replacing their reflection in the mirror.
Live Action TV
The Supernatural episode "Bloody Mary" had this, as it was about a variant of the Bloody Mary Urban Legend mentioned under Real Life. However, this Bloody Mary didn't always kill the person who summoned her: She instead kills someone with a secret that she feels makes them responsible for a death.
Part of the premise of Kamen Rider Ryuki, where Mirror Monsters are simply animalistic creatures that hunt humans and eat them. Some are contracted by the Riders to fight other Riders and Mirror Monsters. While some of the monsters are heroic, they still need to be fed, or they'll eat their contractor. Though they'll wind up eating them if the contract breaks.
Ryuki's American adaptation, Kamen Rider Dragon Knight, is near identical except the heroes are never at risk of being eaten by the monsters they're contracted to. There's still scores of evil mirror monsters to deal with though. That and they're don't eat people but rather capture them.
Doctor Who, "Family of Blood": The family wanted to live forever, and the Doctor granted them that in different ways. He traps the family's youngest daughter inside every mirror. "If ever you look at your reflection and see something move behind you just for a second, that's her. That's always her."
According to Grover on Sesame Street, seeing a monster in the mirror is not a cause for freaking out but an occasion to sing "Wubba wubba wubba wubba woo woo woo." After all, "That monster in the mirror, he just might be you."
The entire premise of Dark Oracle is that the kids' Evil Twins can reach them by coming out through the mirrors. To say this leaves everyone incredibly paranoid would be an understatement.
In Heroes Nikki's alternate personality Jessica would always manifest in reflections and when she takes control over Nikki's body early on, it's Nikki who gets trapped and slamming against the glass. Before she was named Jessica, her Fandom Nickname was ikkiN.
During the WCW version of the Hulk Hogan/Warrior feud, one infamous segment involved Warrior appearing in the mirror of Hogan's dressing room. The idea was for the Warrior to only be visible to Hogan, as Eric Bischoff doesn't notice. The only problem with this: the Warrior was clearly visible to the commentators and people watching on TV.
Call of Cthulhu supplement The Dreamlands, adventure "Lemon Sails". The Temple of the Oracle on Sarrub has a mirror which has been taken over by a wendigo-demon that attacks anyone who tries to use it.
In Forgotten Realms, Turmish even got the law that prohibits large mirrors for this reason.
The Planar Handbook features optional "Mirror Planes", each of which is a plane of existence consisting solely of a featureless corridor and a lot of portals connected to various mirrors all over the multiverse. Entering a Mirror Plane, however, causes an opposite-aligned clone to manifest within, which will attempt to Kill and Replace the original, since there's nothing to do in the Mirror Plane itself.
Exalted gives us Szoreny, possibly the Mirror Monster. One of the Yozis, he used to represent The World Tree until he got turned inside out and became a grove of reflective trees laced with quicksilver. While his Charms haven't be statted up yet, it's theorized a lot of them would center around mirrors, reflection, and unseen connections.
The New World of Darkness has two variations of this. The Urban Legends sourcebook gives various rules about the homicidal ghost Bloody Mary, mentioned in the real life section. Secondly, there's a Vampire specific ghost named Red Jack mentioned in the 'Mythologies' sourcebook that functions as a Bloody Mary for vampires specifically. He is much, muchworse.
Walking with Shadows features a ghost in a mirror who tries to tempt a teenage girl to suicide.
In the game Ghost Master, there are several ghosts who can be anchored to mirrors, and can scare mortals who look into them.
Paranoia from Castlevania: Dawn of Sorrow lives in the mirrors in its boss room, and jumps out of them to attack Soma. Defeating it lets Soma walk through mirrors.
Moreover, the first time you fight Paranoia, you really just fight its reflection. Stepping into the next room reveals even more mirrors... and a much larger Paranoia.
In Realms of the Haunting a literal example appears in one of the trials of Sheol where you have to smash a perimeter of mirrors. But if you look directly into them a monster will pop out of it and chase you. The only way to defeat it is to have it reflected in the mirror and break it.
At two points in Splatterhouse, Rick walks down a hall of mirrors. Several of them have his reflection turn, climb out of the mirror and attack him.
Saltim, a boss from Billy Hatcher is this, he jumps in and out of mirrors trying to attack you and will even try to suck you in.
In Clock Tower: The First Fear, sometimes a hand will reach out of the mirror in the bedroom and try to strangle you when you check it. It's instant death if you haven't learned to reflexively mash the panic button in these types of situations.
In Enchanted Sceptres, there's a dead end with a mirror, which seems "as strong as steel" at first. A Black Knight sometimes appears out of the mirror, and if you smash it then, the knight "shatters into a thousand pieces".
At one point in Prince of Persia, your path is suddenly blocked by a mirror. When you jump through it, your reflection separates from you (you re-merge with it near the end of the game).
Silent Hill 3 features one scare in which you pass through a door into a room that contains nothing but a mirror covering one wall. You cannot interact with, and the door does not open if you attempt to leave. As you move around the room, your reflection suddenly stops. A dark liquid similar to blood then begins to spread around the floor in a vein pattern, spreading across your frozen doppelganger. This is never explained or mentioned again once you escape the room.
Hang around for a while longer, and Heather will die. Fortunately, once your doppelganger freezes the doors to the room unlock and you are free to leave - provided your interest in finding what that room is about doesn't induce you to stay (there's no signal letting you know the door is unlocked).
In Nan Quest, You can see Henry's head in the mirror behind him. Then he moves out of the way of the mirror. Mirror Henry doesn't. And then he turns around, smiling.
Subverted in the SCP Foundation, with SCP-919. If a person's image is reflected in the mirror for 15 seconds, the reflected image starts moving independently of the person... and begins pleading and begging for the person to not leave, since if the person leaves the reflection dies.
South Park had a version of "Bloody Mary" where, if you say his name three times while looking in a mirror, Biggie Smalls will appear in the room with you (understandably pissed because everyone keeps summoning him while he's trying to get to Satan's Halloween/birthday party).
In an episode of Regular Show called "Jinx", Rigby tries to break his jinx by performing a ritual in front of a mirror. He ends up summoning a demonic reflection from inside a mirror named Ybgir. It's banished back when someone chants its name three times. This has a lot of refrences to the Bloody Mary myth.
Kids play a game called "Bloody Mary", chanting the name in front of a mirror in a dark room. Supposedly, her face replaces the reflection in the mirror, and naturally, there are urban legends about kids who kept it up for too long, made Mary angry, and died the next day.
There is a neurological disorder whereby the afflicted cannot recognize their own reflection. While they are in no danger from said reflection, they are left with the terrifying impression that they are being followed by a stranger that has somehow gotten into their house.