It is a Zombie riding a Wither Skeleton Skull.
Have Fun!
Note: Minify the command to avoid issues :)
Factions Villager For Your Factions Server
Potion of Haste
Cursed Training Dummy
Lapis Archer
Pet ENDER DRAGON!!!!!
Wither Storm Boss in ONE COMMAND
HEROBRINE COMMAND
HEROBRINE BOSS BATTLE (See YouTube for details)