The werewolf of Fever Swamp - R. L. Stine

They're baa-ack! Make way for the bestselling children's series of
all time! With a fresh new look, GOOSEBUMPS is set to scare a whole
new generation of kids. So reader beware--you're in for a scare!
There is something weird happening in Fever Swamp. Something really
horrible. It started with the strange howling at night. Then there
was the dead rabbit. 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?