A rat king is a cluster of rats which have become attached to each other by their tails. There is some debate over whether or not the rat king phenomenon is real, with some people arguing that the specimens on display are fakes, while others have attempted to prove that a rat king could actually form under certain conditions. As a general rule, rat kings are viewed as cryptozoological in origin, meaning that while they could potentially be real, they are not widely accepted by most scientists and biologists.

A brown rat.

According to people who believe that rat kings could actually happen, they are created when a colony of rats is crowded together in a cramped space, with the animals facing outwards to deal with potential threats. As a result of prolonged confinement, the tails of the animals start to become attached to each other, entangling and forming knots which are glued together with the body fluids of the animals. When the rats are released from the confined space and they try to pull apart, the knots tighten, permanently connecting the rats to each other.

Several museums have rat kings in their archives or on display. According to people who have studied these specimens, there are some features of these rat kings which suggest that they could be genuine. These features include tails with a large number of breaks, and calluses from where the tails rubbed against each other while the rats were alive. These rat kings are often mummified by dry conditions in walls, further supporting the idea that they formed as a result of being forced into a confined space.

The rat king was viewed historically as a bad omen, and probably with good cause. Rats carry a number of diseases, perhaps most notably plague, so it is understandable that people would associate bad luck with a large cluster of rats. Diseases tend to arise more readily when animals are confined close together, so the rat king would be a breeding ground of disease if it actually existed.

There is some debate over the origin of the name. Medieval scholars have suggested that people may have been under the impression that the rat king was actually one animal with many bodies, with the “king” referring to the animal's size. Legends have suggested that a “king rat” sat on the tails of the rat king, overseeing the movement of the animals as they attempted to navigate with their matted tails.