"Mischief" is the designation for a group of rats. "Colony," "plague," "pack" and "swarm" are also suitable names. A rat king is an unusual phenomenon where a large number of rats become intertwined by their tails due to extreme conditions and inordinate amounts of blood, dirt and excrement.

The rat king was seen as a bad omen in Europe through medieval times and into the 18th century, when cases eventually diminished due to the emergence of the brown rat. In some instances, physical evidence shows that rat kings could survive for extended periods of time despite their obvious physical disadvantages. This is seen in some mummified examples in which calluses have formed around the fractures of the tails.