Rat kings are phenomena said to arise when a number of rats become intertwined at their tails. Historically, there are various superstitions surrounding rat kings, and they were often seen as an extremely bad omen, particularly associated with plagues.
"The Rat King is a cryptozoological phenomenon that occurs when a group of rats become permanently bound to one another by knotted tails, excrement, and hair. The rats thus move and function as one entity. While biologists generally believe the Rat King to be little more than Germanic folklore, specimens of dead Rat Kings are housed in museums throughout northern Europe, the largest of which contains 27 rats. I find even the possibility of a Rat King's existence very disturbing...."