Minecraft wiki entry for Iron, associated text: Iron Golem − a neutral mob, that protects villagers or the player that spawned them. If spawned by villagers and a player hits a villager or the iron Golem, the iron golem targets the player. An iron golem drops 3-5 iron Ingot when killed.
Zombie − Zombies are common mobs that have a 0.83% chance to drop an iron ingot when killed.