Mojang decided that no gravity mobs can't move on their own, so gg.
Note: Minify the command to avoid issues :)
Skeleton Sheriff
Budget bandit
Grab bag of commands used for always active repeating command blocks
XP spawner
Pet ENDER DRAGON!!!!!
Wither Storm Boss in ONE COMMAND
HEROBRINE COMMAND
HEROBRINE BOSS BATTLE (See YouTube for details)