Minecraft wiki entry for Development_versions, associated text: Occasionally, zombies, skeletons, and zombie pigmen will spawn with the ability to pick up items on the ground.
Armor will automatically equip. Items will instantly be held, which is not exclusive to weapons. This means these zombies can be found holding items they have come across, such as rotten flesh.
This will cause zombie pigmen to drop their sword.
This will cause skeleton to drop their bow.
The attribute applied to allow them to pick up items is called CanPickUpLoot.