The Rat King appears only on his level in SLASH'EM. Although all other r are tiny animals, he is in fact medium sized and able to use weapons, wands and scrolls. Oddly enough, despite being a unique monster with his own level, he does not have the G_NOGEN flag.
He can be quite dangerous to the unwary adventurer, owing mainly to the fact that he is reasonably fast, has a lot of hit points, and is always armed with a long sword, meaning his weapon attacks hit relatively hard. He can also use any wands or other magic items that are found in his room, making him potentially all the more dangerous.
More rarely, he can even wield the Vorpal blade. Note that in slash'em the Vorpal blade has 10% probability of instadeath by decapitation instead of 5%!!! In that case, you should not engage in melee, and take him out with ranged weapons. Don't forget to pick it up and stash it before some other monster picks it up.
In biology, a rat king refers to a group of rats, that are accidentally knotted and sticked together at their tails and sometimes legs. It shares some similarities with siamese siblings, but as opposed to actual siamese twins the rats are joined postnataly. Radiographical images show, that the tails of the joined rats had fractures in their tails which healed, implying the rats were alive in that state for a prolonged time.
The more literal Rat King as a large humanoid rat with a crown appears in various works of fiction. The most famous interpretation, is in the ballet The Nutcracker.