If You've Never Heard Of Rat Kings Before, Welcome To Your Worst Nightmare
No, it does not mean a leader of rats with royal heritage.
You've probably wondered, "Is there anything grosser on the planet than a rat?"
The answer is yes: many rats. Many rats are grosser than one rat.
Unfortunately, it gets EVEN WORSE.
The only thing even grosser than an orderly row of rats, or a bunch of well-delineated rats all in the same general vicinity, is A BUNCH OF RATS WHOSE TAILS HAVE BEEN KNOTTED AND FUSED TOGETHER.
I'm warning you.
You don't want to see this. (But you have to.)
It's so bad.
You're going to hate it.
It's REALLY gross, I am NOT joking.
MEET THE RAT KING.
"Rat king" is the name given the phenomena in which a group of rats' tails knot or fuse together with blood, excrement, ice, or dirt, thereby creating a sort of many-headed MONSTER.
The term was originally devised in Germany, by the name Rattenkönig. (In French they're called "roi de rats," which, even if you don't speak French you can tell that's something you don't want anything to do with. You see "roi de rats" and right away you're like, "Pass.") The term originally referred to lecherous persons, but has adapted over time to refer to the abomination of nature you see here.
The above specimen, comprised of 32 rats, was found in a miller's fireplace in Germany in 1828. It is now being preserved at the museum Mauritianum in Altenburg.
Between 35-50 rat kings have been found in history — that we know of. There is probably at least one under your bed right now.