Rat kings are cryptozoological phenomena said to arise when a number of rats become intertwined at their tails, which become stuck together with blood, dirt, and excrement. The animals consequently grow together while joined at the tails, which are often broken. The phenomenon is particularly associated with Germany, where the majority of instances have been reported.
Most researchers presume the creatures are legendary and that all supposed physical evidence is hoaxed, such as mummified groups of dead rats with their tails tied together. Reports of living specimens remain unsubstantiated. One theoretical cause for the phenomenon is cramped living space; young rats might live too closely together, becoming inextricably entangled. However, the normal behavior of rats, which generally seek their own comfort, speaks against this theory. No scientific study has been performed to prove a natural cause of the phenomenon.
Specimens of purported rat kings are kept in some museums. The museum Mauritianum in Altenburg (Thuringia) shows the largest well-known mummified "rat king", which was found in 1828 in a miller’s fireplace at Buchheim. It consists of 32 rats. Alcohol-preserved rat kings are shown in museums in Hamburg, Hamelin, Göttingen, and Stuttgart. A rat king found in 1930 in New Zealand, displayed in the Otago Museum in Dunedin, was composed of immature Rattus rattus whose tails were entangled by horse hair. Relatively few rat kings have been discovered; depending on the source, the number of reported instances varies between 35 and 50 finds.
The earliest report of rat kings comes from 1564. If real, the phenomenon may have diminished when the brown rat (Rattus norvegicus) displaced the black rat (R. rattus) in the 18th century. Sightings have been sporadic in the modern era; most recently comes an Estonian farmer’s discovery in the Võrumaa region on January 16, 2005.
Most extant examples are formed from black rats (R. rattus). The only find involving sawah rats (Rattus rattus brevicaudatus) occurred on March 23, 1918, in Bogor on Java, where a rat king of ten young field rats was found. Similar attachments have been reported in other species: in April 1929, a group of young forest mice (Apodemus sylvaticus) was reported in Holstein; and there have been reports of squirrel kings. The Tartu Ulikooli Zooloogiamuuseum (Museum of Zoology in Tartu, Estonia) has a specimen. The Zoological Institute of the University of Hamburg allegedly owns a specimen.
The rat king discovered in 1963 by the farmer P. van Nijnatten at Rucphen (Netherlands) as published by cryptozoologist M. Schneider consists of seven rats. X-ray images show formations of callus at the fractures of their tails which according to proponents show that the animals survived an extended period of time with the tails tangled. 
Historically, rat kings were seen as an extremely bad omen, particularly associated with plagues. This is a reasonable conclusion if they are formed naturally, since large populations of rats housed in insufficient space generally bring with them disease and pestilence. With an increase in the size of the rat population comes an increase of the chance of outbreak of disease — for example, the Black Death, which is spread by the rats’ fleas.
The term rat king has often led to the misconception of a king of rats. This idea has been particularly appealing to the literary and artistic mindset: The Nutcracker, by Pyotr Ilyich Tchaikovsky, adapts a tale by E. T. A. Hoffmann that features a seven-headed Mouse King as the villain. Another example is the fairy tale Rattenkönig Birlibi, by Ernst Moritz Arndt.
Today, the rat king is occasionally used in novels such as China Miéville’s King Rat, The Tale of One Bad Rat by Brian Talbot, Peeps by Scott Westerfeld, or Rats and Gargoyles by Mary Gentle. A fictional treatment of the rat-king legend and the supposed power over other rats (and humans) can be found in Terry Pratchett’s novel The Amazing Maurice and his Educated Rodents. One recent reference to the rat king can be found in Lars von Trier’s film Epidemic, with the rat king as an omen for the upcoming disease. The concept also forms a theme in Michael Dibdin’s detective novel Ratking. In Alan Moore’s and Ian Gibson’s comic book series Halo Jones, the Rat King was a weapon of war, a super-intelligent collective of five rats with entwined tails who were able to communicate via a computer terminal.
In the episode "Jack Meets Dennis" of NBC’s 30 Rock, Liz’s sleazy boyfriend Dennis, when describing their date, says they saw a rat king and goes on to describe, with Frank’s help, the definition of a rat king, multiple rats with permanently entwined tails. Later, Liz realizes, with Jack’s help, that Dennis is a metaphorical rat king and the longer she stays with him the more inextricably they will become entwined and therefore she must immediately "exterminate" (break up with) him.
In the Stephen King novel It, a rat king is seen by Richie in a cupboard. Although he does not refer to it by name, he describes it as follows: "There’s hundreds of them in there... Their tails... they were all tangled up, Bill. Knotted together... Like snakes."
In 1993, German artist Katharina Fritsch exhibited a large scale sculpture/installation titled "Rattenkönig" (Rat-King) at the Dia Center for the Arts in New York City