I may be wrong, but rat kings seem like one of those cryptids whose reputation, like the chupacabra’s, exploded thanks to the Internet. One minute I’d never heard of the things, the next minute they were all I heard about.
That said, they've got a rich history in research, folklore, and fiction and, more importantly, they are totes creepy at the gaming table. A) They’re a bunch of rats knotted together at the tail, which is just: How? and: Why? and: Gross! B) That means five bite attacks. All diseased. With bubonic plague, naturally. C) They're intelligent (Int 5), thanks to a communal mind shared by the tangle of bodies. D) Oh, and they can communicate with other rats, rat swarms, and dire rats…and boost their diseases, too.
But the real danger of rat kings is the fear they inspire. To city dwellers in the poorer quarters, they are harbingers of plague, corruption, and death…and so the sight of one can inspire fear and social unrest long after the original rat king is dead.
After a rat king is seen in Spear Square, the port city of Tamar fears plague and deprivation will follow. The paranoia also leads to attacks on ratfolk and harassment of sailors and other travelers, which hurts trade. The lord mayor wants the unrest quelled, and that starts with a good old-fashioned sewer cleanout to slay as many rat kings as possible.
After adventurers make their first foray into the Undercity, a rat king begins paying secret nightly calls to any rat familiars or dire rat companions they might have. The rat king in no way threatens the adventurers, and may even drop a crumb or two of useful information. The rat king has plans for its subterranean domain—plans clumsy adventurers could thwart—and it hopes to bring the party’s rat over to its side…or at least sow doubt and confusion so the rat hesitates at a critical moment.
A rat-catching bard uses magic and alchemical brews to create rat kings, which he then uses to burgle the rich. The money and goods he steals are only a means, though, not an end. His final plan is a mystery, but adventurers attempting to stop him will have to face other conjoined horrors, including giants made of people fused together by their own half-melted flesh (treat as Advanced ettins).
—Pathfinder Bestiary 4 225