Items, NPCs, quests, objects and spells of patch 1.12.1, read live from the realm itself.

Nether Wing

NPC	Level	Zone	Chance
Nether Maiden	37 - 38	Desolace	80.0%
Nether Sister	38 - 39	Desolace	80.0%
Nether Sorceress	39 - 40	Desolace	80.0%