Easy it's just a Ghast Riding a GIANT
Note: Minify the command to avoid issues :)
Cool Holographic Text (No more floating horses)
Notch Book Troll
Killer Blaze Rod
Hider Killer
Pet ENDER DRAGON!!!!!
Wither Storm Boss in ONE COMMAND
HEROBRINE COMMAND
HEROBRINE BOSS BATTLE (See YouTube for details)