Java Edition:Mob Wearing Wool
Mobs normally cannot wear wool. However, there has been a method to get skeletons, zombies, and zombie pigmen to wear wool.
Obtaining
In the April Fools' version 2.0 red and purple, every 40 to 90 seconds "Super Hostile Mode" will activate. When this event happens, many mobs including creepers, skeletons, zombie, and zombie pigman, and occasionally blazes, will appear while lightning strikes in the area, potentially creating charged creepers. Skeletons, zombies, and zombie pigmen have a 25% chance to spawn wearing wool on their heads, which can be one of the 16 available wool colors.
All skeletons, zombies, and zombie pigmen that spawn from this event will be wearing enchanted armor of either chainmail, iron, or diamond. Skeletons will spawn with enchanted bows, zombies with enchanted diamond swords, and zombie pigmen with a battlesign. All mobs that spawn from this will have the CanPickUpLoot data tag set to false, preventing them from picking up items. Skeletons that spawn riding spiders, or spider jockeys, are also affected by this.
Retaining
Retaining this feature is difficult, mobs that spawn from the "Super Hostile Mode" event will disappear after about 40 seconds, unloading the chunks make them disappear, and closing the world will make them disappear. The only known method of saving these mobs is to send them through a portal, either a nether portal or end portal, will prevent them from disappearing. However, this is a temporary solution as these mobs can still despawn like other mobs without persistence.
To permanently retain these mobs, the player will have to prevent these mobs from despawning and then update to 13w16b (1.6 Snapshot), the version that added name tags. Until then, the player can attempt to unload these mobs. First, by reducing the render distance to "tiny" before leaving, which will increase the chance that the mobs will be unloaded that than despawn, or by leaving through a portal which will unload the chunks when changing dimensions.
Alternatively, the player can update straight to 13w16b. However, this will prevent certain discontinued features from being obtainable.