Nothing drives players crazier than not being to identify their foes. Or at least the players that go out and buy every book and study them before they play. Unlike video games people don't have all their info floating over their head. Giving the players a description of what they see instead of telling them will force them to try to figure out how to proceed. I used this trick a lot when running World of Darkness where the guy turning into a wolf could be a talented magician. a vampire or even a werewolf.