Mental Floss:

A ball of furry fury, a rat king occurs when the tails of rodents become twisted, wrapped, and warped into a knot so impossible that not even the world’s most loyal Boy Scout could untangle it. Rat kings have been reported since the mid-16th century (almost entirely within Germany), and everything about them—from their name, to their cause, to their very existence—remains suspended in mystery.

RTWT