Minecraft wiki entry for Drops, associated text: All items the mob picked up after spawning are always dropped, even when it isn't killed by a player and even if it is a baby zombie.
This is the case only for hostile mobs, not for villagers.
Ominous banner always dropped by illagers wearing it.
Horses, donkeys, mules, llamas and pigs, and striders drop any items equipped to them by players, like saddles, carpet or chests.
Ravagers drop their saddle.
Drowned drop nautilus shells spawned in the offhand.