The horned devil is flagged to only appear in Gehennom, but will not be randomly generated there since it is lawful. This trait also occurs with barbed devils and the erinys - all three are still eligible to be generated via gating by other major demons. Horned devils can also be summoned if the summon nasties monster spell is cast in Gehennom or by a lawful spellcaster. Horned devils also serve as minions for chaotic gods and Moloch.
Horned devils may be generated in graveyards. In addition, the lairs of Asmodeus and Baalzebub will each contain a horned devil upon normal level creation. Another horned devil appears within Moloch's Sanctum, placed just outside the temple near its southeast corner.
The horned devil is considered the weakest of the major demons by the game, with lower monster difficulty and hit die but better AC than the barbed devil. It also has a lower difficulty than the water demon, who has an AC closer to the horned devil's but also possesses weaker hit die; this is likely because the water demon is faster and can swim.
Horned devils are slower than an unhasted player, and a fast well-armored character with a well-enchanted blessed and/or silver weapon will have relatively little trouble dispatching one - a wand or cone of cold can eliminate them in one or two shots. However, they can still pose a significant threat if they disarm you with their bullwhip, which speedrunners in particular will find a major nuisance - having Excalibur yanked out of your hand, e.g. in the middle of a pitched fight against Asmodeus or Baalzebub, can be fatal.
Horned devils are a decent source of tridents and bullwhips for those looking to utilize either.
The horned devil is introduced in NetHack 3.0.0.
In SLASH'EM, chaotic players have a chance of receiving a horned devil minion as a servant from sacrificing while at any experience level ranging from 10 to 16.
In SpliceHack, horned devils are given a passive quill attack.
In EvilHack, The Idol of Moloch is an artifact figurine of a horned devil and the Infidel quest artifact. It can be applied like a normal figurine to summon a random major demon, including a horned devil - this does not use up the artifact.
These devils lack any real special abilities, though they
are quite difficult to kill.