Dragon (monster)
A dragon, D, is a monster that appears in older versions of NetHack from Hack 1.0 to NetHack 2.2a.
A dragon has a single attack and a fire breath weapon.
A dragon corpse is poisonous to eat, and eating its corpse always grants fire resistance.
Generation
Randomly generated dragons are typically created hostile.
Dragons are among the monsters that can be generated in throne rooms.[1]
Strategy
Dragons are highly dangerous monsters with vicious melee attacks that can also use their breath weapon in melee range, but a hero that is able to overcome one and has poison resistance can secure fire resistance for Hell.
Black dragon breath causes instadeath, and there is no reflection, disintegration resistance or amulet of life saving, making genocide tempting; this is counterbalanced with fire resistance being necessary to traverse Hell, and on top of a hero waiting until they can obtain the resistance to use genocide, there is no means of genociding only black dragons.
Later versions
NetHack 2.3e has different colors of dragon, which are each associated with a specific element and have a corresponding breath weapon—they are not yet given distinct monster entries, and their corpses behave as a normal dead dragon when eaten, i.e. they are poisonous and always grant fire resistance. NetHack 3.0.0 establishes the dragon monster class by phasing out the dragon monster and giving all of the modern dragon types their own distinct monster entries, along with baby forms and the ability to make dragon scale mail by polymorphing their corpse (and later by enchanting dragon scales).