The shapeshifters or shape changers include:
All are capable of imitating other monsters - which monsters they imitate is dependent on which monster is in question - and will shift between disguises on a frequent basis. Wearing a ring of protection from shape changers will force them into their normal form. Forcing one to polymorph (by wand, spell, or potion) will not change their base form - they'll polymorph immediately, but continue to shapeshift. Forcing one to polymorph while it is mode-locked by protection from shape changers can change its base form permanently. They are created with starting inventory appropriate for their polymorphed form.
Chameleons, doppelgangers and sandestins have 1/6 chance every turn to change shape. Chameleons choose an animal form 2/3 times, doppelgangers choose a "nasty" form 1/7, or if that fails, then choose a player class (2/3 chance). Sandestins pick a "nasty" form 6/7 times.
Lycanthropes are also a type of shape changer, alternating between human and animal. They are:
Finally, we have the mimics. They do not attempt to change into other creatures, but they instead pretend to be objects, doors, or stairs, and most often reside in shops. As far as actually fooling players goes, these are by far the most effective. There are three variations: