Rat kings involve a number of rats intertwined at their tails, which become stuck together with blood, dirt, ice, or feces – or simply knotted. The animals reputedly grow together while joined at the tails. The numbers of rats joined together can vary, but rat kings formed naturally from a large number of rats occur more rarely. The phenomenon is particularly associated with Germany, which may have produced the majority of reported instances. Historically, various superstitions surround rat kings, and they were often seen as a bad omen, particularly associated with plagues.

http://en.wikipedia.org/wiki/Rat_king_%28folklore%29