A wither skeleton called the necromancer, drops some good stuff too.
Note: Minify the command to avoid issues :)
The Necromancer
FLOATING SQUID!!!!!!
Potion of lava swimming.
Underpants of strength and revenge
Pet ENDER DRAGON!!!!!
Wither Storm Boss in ONE COMMAND
HEROBRINE COMMAND
HEROBRINE BOSS BATTLE (See YouTube for details)