[Taqwus]

Hm, interesting.

If there were shapechanging on other actions -- shapechanging right before or after it makes its first melee attack of a battle, say -- one could model even funkier behaviors; e.g. a gargantuan armadillo being nigh-invulnerable until it rears up and starts stomping, thus revealing its somewhat more penetrable underside.

Or even odder things, like seriously irritating flying raiders that charge in, but who get a far lower morale once they've attacked and thus fly away shortly thereafter.