Minecraft wiki entry for Helmet, associated text: If a zombie, husk, stray or skeleton is wearing armor, there is a 8.5% chance (9.5% with Looting I, 10.5% with Looting II and 11.5% with Looting III) for the mob to drop a helmet upon death. The dropped helmet is usually badly damaged, and may be enchanted with enchantment levels 5-19.