The bone devil is only randomly generated in Gehennom, and is also a valid form for a polymorph trap, which may result in it appearing much earlier. Bone devils are also eligible for gating by other major demons.
Bone devils are decently fast, with a weapon attack and a poisonous sting that can drain strength unless the player has poison resistance. Like most demons, it is vulnerable to silver and cold attacks.
However, their -1 AC pales somewhat in comparison to other major demons such as horned devils, and they are not particularly threatening to an average player that has reached Gehennom. The bone devil's low AC can still make it a potential nasty surprise for earlier players if another hostile monster is polymorphed into one, unless they are sufficiently prepared and have poison resistance.
As a potential form for pets led onto polymorph traps, a bone devil is a decent tame monster to have, although the below-average magic resistance may interfere with further attempts to change its form if you are aiming for something else.
Bone devils attack with weapons and with a great hooked tail
which causes a loss of strength to those they sting.