Banksy is, as evidenced by this list, a big fan of using rats in his art. As with everything surrounding the mysterious artist, no one knows for sure why, but it's been theorized that the creature represents Banksy himself, a cunning figure who comes out at night.
It also seems to serve as an homage to French graffiti artist, and Banksy idol, Blek le Rat.
The artist himself reportedly once said of rats, “If you feel dirty, insignificant or unloved, then rats are a good role model. They exist without permission, they have no respect for the hierarchy of society, and they have sex 50 times a day.”
This particular little rat, ready for the cinema, was discovered after the Sundance Festival in 2010. A little worse for wear, it is currently being protected by the theater.