A monster in Nethack. Represented by a blue '&'.
Water demons can be very good, but most of the time, they are very nasty. They can be found from fountains randomly. Drinking from fountains or #dipping stuff into fountains sometimes releases them (that often does something much nastier though - such as releases water nymphs, or tons of water moccassins...)
Water demons are often in very bad mood if you release them. However, sometimes they are very generous and grant you a wish. This is one of the easiest (?) ways to get a blessed +2 gray dragon scale mail on an early level, but requires extremely good luck...