Minecraft wiki entry for Renewable_resource, associated text: Most items dropped by animals or monsters are renewable because more mobs of the same kind can spawn and be killed again for more loot. The exceptions are drops of enemies that spawn just once with the generation of specific structures, such as elder guardians (three spawn with each ocean monument). Drops of hostile mobs, with the exception of those obtainable by other means, are not renewable on Peaceful difficulty because the monsters do not spawn.