Sometimes a bunch of rats get their tails tangled into a big, nasty knot. As the legend goes, these rats form a sort of hive mind, which can approach human-level intelligence. A "rat king" is what you call either this collection of rats as a unit or the single rat that controls the group. In lots of stories rats cluster together into the shape of a much larger rat, and this is the Rat King. Creepy stuff, no?