Java Edition:Wool Headed Mob
Zombies, zombie pigmen, and skeletons can wear wool on their head in the April Fools' update 2.0. The wool can be any color.
Obtaining
In 2.0 red and 2.0 purple, every 40 to 90 seconds super hostile mob would activate. When this occurs, creepers, zombies, skeletons, and zombie pigmen appear and lightning strikes them. This allows several discontinued mobs to spawn including mobs wearing wool on their head. The zombies, skeletons, and zombie pigmen have a 25% chance for wool on their helmet slot. The wool is evenly distributed among the 16 colors. There is no other method to get wool on the mobs' heads. All wool headed mobs must have enchanted armor that is either chainmail, iron, or diamond. If it is a zombie pigman it must be holding a battle sign, zombie must be holding an enchanted diamond sword, and skeletons must be holding an enchanted bow. All super hostile mobs can not pick up loot. Spiders that become spider jockeys can have wool headed skeleton passengers.
Wool Headed Spider Jockey
In these versions, spiders that become spider jockeys can have wool headed skeleton passengers. Like the other mobs, the skeleton passenger can have any color of wool in its head.
Retaining
Retaining this feature in a world is very difficult. The super hostile mobs disappear after approximately 40 seconds. Closing the world will make all of the super hostile mobs instantly disappear. Unloading the chunks will also make the super hostile mobs disappear. The only known method to stop the mobs from disappearing is sending them through a portal. Any mob sent through an end portal or nether portal in any dimension will not disappear. That fixes the temporary problem but they will still be able to despawn normally like other mobs. This is until 13w16b (1.6 snapshot) when the name tag was introduced. There are two workarounds to the fact that they'll despawn. First, leaving the mob in unloaded chunks. Mobs can't despawn in unloaded chunks so storing them in a location and then never returning until name tags is an effective way to keep them. When leaving the chunks to make them unloaded it is suggested to bring the render distance to "tiny" because this will increase the chance the mob will be unloaded rather than despawned. The other option is to exit via a portal and unload all the chunks when changing dimensions. The second option is to upgrade straight from 2.0 to 13w16b with name tags then get one on the spot. This method will not allow for any discontinued features during this period.