Rat king The “rat king” forms when a number of rats become entwined by their tails - either knotted or tangled together with blood and excrement. The resulting cluster is said to have been observed since the 16th c. and was associated with the plague. There have been several dozen sightings of as many as 32 rats joined like this and a handful of specimens preserved in museums, with the most recent report out of Estonia in 2005