This actually needs two command blocks.
Note: Minify the command to avoid issues :)
walcom hom jimmy
NEW AND IMPROVED LITTLE PUFF!!! (Little Puff 2.0)
#NiceHat
Bill the Purple Man
Pet ENDER DRAGON!!!!!
Wither Storm Boss in ONE COMMAND
HEROBRINE COMMAND
HEROBRINE BOSS BATTLE (See YouTube for details)