Cranium rats are psychic rodents with exposed brains who share a hivemind. Created by mind flayers, when they come together to form a swarm, they merge their minds into a single intelligence. Strix has a pet cranium rat named Stinky.
Cranium rat statistics can be found in Volo's Guide to Monsters, page 133.
Cranium rats first appeared in the Planescape setting. In the Faction War supplement, it is said that there are four different cranium rat hiveminds operating in Sigil. The most prominent of the hiveminds is known as "Many-as-One", or "The Us".