This seems to be a fairly recent development. Nobody knows really why he became a bear. But instead of being a guitar guy, Petrucci is now a guitar bear.
Be wary: not only can Petrucci kill with his massive mauling claws and razor-sharp teeth, but he can also unleash a brain-jellying, nerve-ripping guitar solo with both paws. It might sound like the guitar's being played with a big velvet hammer...but now imagine that big velvet hammer smashing your skull in.
Beware of John Petrucci. Avoid him at all costs.