Freddy and the Ignormus by Walter R. Brooks

A mysterious and fearsome creature called the Ignormus is living in the Big Woods. No one has actually gotten a good look at it but, because of the Ignormus, the animals live in terror of going into the Big Woods. Freddy the Pig makes the mistake of borrowing Mr. Bean's gun without permission for protection in the Big Woods and loses it. To make matters worse, someone leaves a bag of stolen oats in the pig pen and everyone, including Mr. Bean, believes that Freddy is the thief.