Beginning in NetHack 3.6.0, vampires and vampire lords are capable of shapeshifting; vampire lords may become vampire bats, fog clouds, and wolves. These forms will resurrect as the vampire lord if they are killed/destroyed. The fog cloud form is notable for being able to pass under locked doors.
Vampire lords make excellent pets if you wear a ring of protection from shape changers. Without the ring, pet vampires will shapeshift. They are weak fighters in bat/wolf/cloud form, and if they are wearing armor, it will be destroyed when they become wolves. But if you do wear a ring (or play a version in which they do not shapeshift), vampire lords can wear all armor in addition to their base AC of 0, which makes them very hard to hit and allows you to give them some resistances. They also do not eat (except when shapeshifted), fly, and regenerate rapidly, and their drain life attack weakens the monster they and you are fighting.