The expression 'rat king' is applied to a group of rats whose tails are tied together. The causes of this rare phenomenon remain uncertain. The article describes Estonian finds of rat kings. This additional information is used for the evaluation of hypotheses concerning the mechanism of rat king formation. Estonian finds support the hypothesis that the rat king's knot is created after the rats' tails are glued or frozen together. Analysis of the worldwide geography of the finds of rat kings demonstrates that with one exception (Java), they occur in regions where two factors coincide: cold winters and the presence of the black rat, Rattus rattus.