Hideous amalgamations of rat and human, they breed with rats, each other, dire rats, and they are obsessed with books, knowledge, and magic. They can innately read magical scrolls and writing, and like to find those things and consume them. They have the ability to cast cause fear and dimension door (at least), and can deal a small amount of sneak attack damage (like a Rogue).
Rats the size of cats, with two-foot-long tails, human hands, and the faces of old humans. Gull encountered a pair of them building nests, but avoided their notice when initially sneaking through the library at Briarstone Asylum in Chapter 12. One of them cast an effective fear spell on Gulliver in Chapter 13, and another delighted in throwing a book at Grip.
In Chapter 14, the party tried negotiating with the ratlings for passage through the rest of the asylum, only to find they had abandoned their nests there.
In Chapters 20 & 21, after working out a truce with their leader, Ratch Mamby, the party allowed them to eat books from the Briarstone library after Dora had finished reading them.
First Reference: Chapter 12
Other Notable References: Chapter 13, Chapter 20, Chapter 21, Chapter 29, Chapter 30