If you happen to be looking for some basic OSR compatible stats for a Rat King, the creature can be found in Casey Sorrow's Bestiary of Fantastic Creatures: Vol 1, find it: here. I happened to get a hard copy of this little zine years ago, I think when it was crowd funded or just available somewhere, and it has a nice collection of weird monsters in it. Note that I have no affiliation with Casey Sorrow or the Bestiary, it is just something I have on my shelf.

And I would use the Rat King (Giant) if you go with this book.