A rat king is the term used to describe an agglomeration of rats whose tails have entangled together. As a result of this entanglement, a large single entity is formed, which is usually further bound by other substances, such as blood, feces or other filth. The concept of this monstrous creature is said to have existed for centuries, and there are a number of specimens in natural history museums that seem to attest to their existence.
Rat King Origins
According to one source, the name ‘rat king’ may have its origins in an old belief which states that elderly rats known for their wisdom would sit on the entangled tails of his fellow rats. This rat was believed to have been treated as royalty by the other rats, hence giving rise to the term ‘rat king’. Interestingly, this phenomenon is not limited to rats alone, but mice and squirrels have also been found occasionally to be caught in large knots. It is unknown, though, if the old folklore applies to these creatures as well.
"Roi des rats" found in 1986 in Vendée, France (CC BY-SA 3.0)