Because rats are social, have long tails, and happily live in dirty environments, they occasionally get stuck together. In particular, their tails can become tangled, and they're unable to separate themselves once this happens—though they usually groom themselves well enough to keep this from happening in the first place, making the phenomenon thankfully rare. As rare as it may be, though, it's spectacular enough that humans—long-time unwilling companions of ratkind—have noticed these unfortunate entities and ascribed them with special properties. Details vary by region, but generally they're held to be omens of plague and other rat-based calamities.
Rat kings in fiction tend to have greater than typical intelligence, even on par with a human being, and the right to command other rats, be it through some conferred rat respect or overt psychic control, potentially turning a dissociated nuisance into a swarming wave of diseased, omnivorous devastation. They may be viewed as the manifestation of a rat Hive Mind, the rodent equivalent of a Hive Queen. Storytellers who haven't done the research or don't like the "several rats conjoined by their tails" image may instead depict Rat Kings as royalty of some sort, typically Rodents of Unusual Size or anthropomorphic Rat Men imbued with greater intelligence and rule over other rats.
Note that some Pest Controllers control rats. In order to count as Rat Kings, they must be rats themselves, or at least very rat-like. Wearing a rat costume doesn't count.
- Alan Moore's serial for 2000 AD, The Ballad of Halo Jones, had the heroine sign up in the Army to fight a deadly war on an alien planet. She discovers the heart of the supercomputer directing Earth's war effort is a malevolent and superintelligent Rat King plugged into the network as its CPU.
- A Rat King is the Big Bad of the Beasts of Burden story "Something Whiskered This Way Comes", leading a massive colony of rats in the sewers beneath the town. and plotting to overthrow mankind with the help of a powerful supernatural entity.
- Invoked in Dungeon Keeper Ami, when the titular character started using trained rats to perform menial tasks.
- Cathy: "Brr, rats. I find them gross. Oh well. If they can do part of my work, I will put up with a horde of rats bustling about the dungeon. However, I see a minor problem."Mercury: "Yes?"Cathy: "All hail the Queen of Rats!"
- The Discworld book The Amazing Maurice and His Educated Rodents has one as its ultimate villain. Spider, so-called because it's made from eight young rats (eight being a number of great occult significance on the Discworld), has psychic powers so potent it can not only control rats and see anything they can see, but also influence human behavior. It can even strip away the magical awareness given to the protagonist rats and cat, making them ordinary creatures. Horribly, making a Rat King is part of the qualification for mastery in the Ratcatchers Guild, suggesting that there are many of these things across the Disc. It's specifically noted that the notion of Rat Kings being formed naturally is complete bullshit, as baby rats have very short tails and the mothers will keep their nests as clean as possible, and the myth that they get tangled together in filth is something invented to hide the fact that it's humans tying them together.
- The Mouse King from The Nutcracker has seven heads, which suggests that it might have been inspired by stories of Rat Kings. He's changed to the Rat King in many adaptations, since rats are seen as more villainous than mice.
- In the Warrior Cats book Firestar's Quest, SkyClan is threatened to be wiped out by a swarm of rats. Firestar realizes that the rats have a leader, which is more intelligent than the others: it is able to speak Cat, and give commands to all the other rats. Once he kills the leader, the rest of the rats have nothing to command them, and they scatter.
- In Beyond The Spiderwick Chronicles the idea of a rat king is discussed when the protagonists find three salamander-like baby dragons with their tails fused together. It's later discovered that the dragons are merging together into a massive Wyrm King, i.e., a hydra.
- In Rats and Gargoyles, the titular Rats are ruled by groups of nine of their number with their tails deliberately fused together.
- In It:
"A litter. Biggest litter I ever saw...anyone ever saw, probably...Their tails...they were all tangled up, Bill. Knotted together. Like snakes."
- In Nancy A. Collins's Urban Fantasy novel Left Hand Magic, a supernatural conglomeration of twelve New York street rats tries to eat the heroine's puppy. Luckily, her witchy boyfriend's familiar instead ends up eating the rat king.
- A somewhat literal take on the trope, in Neverwhere, the nations of London Below include a highly regarded class of humans called Rat Speakers, who carry out the orders of the rats, who themselves are ruled with absolute authority by an entity described only as "The Golden".
- The Redwall novel Triss features an even nastier variant using vipers instead of rats. The trio of newborn adders got their tails ensnared in the chain of a flail used by an enemy who'd killed their mother and, having no hands, couldn't untangle themselves.
- The primary antagonist(s) of Sterling E. Lanier's The War for the Lot is a blind, psychic Hive Mind rat-king.
- Invoked by David in Animorphs. When he reappears twenty-odd books after being trapped as a rat and left on an island, he claims that rats see him as a natural leader and obey him, that he has armies billions strong. Rachel is skeptical and soon says that that's not possible, rats are not like that and he can't communicate with them that way and something else is going on. She's right.
- Discussed on QI, as well as the similar Squirrel Kings. Jeremy Clarkson finds the idea hysterical, in light of the damage grey squirrels have done to British woodlands, although he admits it's a bit sad that it usually happens to baby squirrels.
- In Dungeons & Dragons' Planescape setting, cranium rats function similarly to rat kings in that they become more intelligent and capable at higher population densities. Enough cranium rats crammed into a small space can rival the most intelligent beings in the multiverse, command other cranium rats to do their bidding, and host a variety of dangerous psychic powers.
- Exalted has emperor rats, also known as "rat-hydras." They come to be when multiple giant rats are fused into one creature by Wyld energies.
- Cataclysm: Dark Days Ahead has Rat Kings as enemies in caves. They can inflict the player with a disease called "Ratting" which reduces stats, can cause vomiting, or, in especially bad cases, cause the player to mutate slowly into a rat.
- In Dark Souls II, a large hyper-intelligent rat actually goes by "the Rat King" moniker, commanding all other rats in the game. You can actually join the Rat King's Covenant—while he bemoans your un-rat-ness, he magnanimously allows you to serve him for some nifty multiplayer perks.
- Diablo III goes for a very unusual take: Rat Kings are large, bipedal rodent-like monsters that carry their young within the swollen flesh on their backs, and can summon said young to attack the player.
- Fallout 2 has "Keeng Ra'at" who has an army of rats to do his bidding and his brother "Brain" who is the leader of a ghoul faction in the town of Gecko.
- In Shadowrun Returns' "Hong Kong" campaign, Gobbet's (a shaman with a rat totem) loyalty mission involves returning to collective she used to live in, in order to confront an old friend (another rat shaman) who's gone toxic and started ruling with an iron fist after acquiring a fetish (no, not that sort!). When you meet her, she's somehow merged with a swarm of rats to become a sort of Eldritch Abomination.
- Little King's Story: The Rat King leads a group consisting of himself and 3 other rats. It and its followers symbolize the Horsemen of the Apocalypse, with the Rat King symbolizing Death.
- Going by the description of Alolan Rattata, it seems as though Alolan Raticate from Pokémon Sun and Moon are this. Not to mention, an Alolan Raticate serves as a Totem Pokemon boss in Pokemon Moon and summons Alolan Rattata to assist it in battle.
- TinkerQuarry: The rats in the Dollhouse are ruled by the Rat King, a massive, three-headed, albino rat that serves as one of the last few bosses of the game.
- In Rat Movie: Mystery of the Mayan Treasure, the rats are lead by "the giant rat that makes all the rules". If the Giant Rat dies, another one grows in size and becomes the new leader.
- In The Penguins of Madagascar, there is a recurring antagonist named "the Rat King". He is a gigantic, muscle-bound rat in command of a gang of sewer rats.
- The Rat King from Teenage Mutant Ninja Turtles tends to be depicted as having control over rats in some adaptations. In the first cartoon, he is initially capable of doing so via a flute ala The Pied Piper of Hamelin, and in later episodes he can do it psychically. In the 2003 and the 2012 cartoons, he is able to do so mentally from the start.
- Batman Beyond has Patrick Poundstone aka Ratboy from the episode "Rats", who can control the giant rats that live below Gotham. He is a Stalker with a Crush to Dana Tan, Terry's Love Interest and kidnaps her. When she rebuffs him, he orders his rats to kill her but Batman manages to save her. Other people were not so lucky.
- The Rat King in the Adventure Time episode "Little Brother" is, of course, the king of all rats, being a normal sized rat with a crown and a cape hiding a legion of rats making him look taller.
- In Hilda (the Netflix adaptation at least) the Rat King is a swarm of rats that move in a hulking mass and speak in unison. They know all secrets overheard by the rats of Trollberg and will happily trade them for more.
- In Family Guy, an "undulating" rat king is listed among the reasons the Quahog Board of Health shut down a Medieval Times-esque restaurant.