Rat Kings are very disturbing entities. They form when a large amount of rats get their tails permanently tangled together.
This mainly occurs during colder periods when rats huddle together. Their tails happen to instinctively want to coil around objects. This in combination with the cold, can cause the tails to freeze together in a knot like pattern. Add on adhesives such as fecal matter and the knot of tails will never be undone.
Eventually the rats will decay and almost certainly all die.