Alright I am brand SPANKIN new here and just started reading/posting...
First off AWESOME idea for a costume
To build something of that size my first thought went to a wire/cardboard frame underneath and like the previous post said hard insulation boards and expandable foam. I have been using more and more of the expandable foam to build odd shaped things because its light weight and carvable. For more of the detail I suggest the insulation boards because for that specific costume you want it wood looking and those boards are in long big ol' pieces and could be made to look like wood fairly easy. For the mouth I would try the expandable foam.
I did a recent project with a set of horns and paper clay over it and when the paper clay dried it cracked but it was a perfect look for the horns and might work for the lips and teeth of your creature.