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. Examining a shapeshifter with a stethoscope will identify it as such.
Chameleons, doppelgangers and sandestins have a 1/6 chance every turn to change shape. Chameleons choose an animal form 2/3 times, doppelgangers choose a "nasty" form 1/7 times, 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:
Since NetHack 3.6.0, vampires can change into several different forms. Unless you wear a ring of protection from shape changers, they will often be seen in vampire bat form, but can also turn into fog clouds to bypass doors. Vampire lords can also turn into wolves. Killing them in any alternate form will cause them to rise in their base vampire form. In 3.6.0 only, due to a bug, genociding vampires would make shapeshifted vampires unkillable.
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: