The Werewolf of Fever Swamp by R. L. Stine
Something weird is happening in Fever Swamp. Something really horrible. It started with the strange howling at night. Then there was the rabbit, torn to shreds. Everyone thinks Grady's new dog is responsible. After all, he looks just like a wolf. And he seems a little on the wild side. But Grady knows his dog is just a regular old dog. And most dogs don't howl at the moon. Or disappear at midnight. Or change into terrifying creatures when the moon is full. Or do they?