These beavers are creatures of the Land of the Dead and one of the perils of the Petrified Forest. They are full of fire and swim in the ponds of tar as if it were water. They are fed bones of lost souls which they gnaw as if they were wood.
However they also seem to be lured by the bones. One thing that can "kill" them is extinguishing them, as with the means of a fire extinguisher.