It is a Zombie riding a Wither Skeleton Skull.
Have Fun!
Note: Minify the command to avoid issues :)
Potion of Haste
Splash Potion of Mining Fatigue
Villager Charger Soldier
Spell Tome of Heath II
Pet ENDER DRAGON!!!!!
Wither Storm Boss in ONE COMMAND
HEROBRINE COMMAND
HEROBRINE BOSS BATTLE (See YouTube for details)