Ever wanted to see how much health a mob has left? Well this command adds health bars over almost every mob so you never have to worry about it again.
Mobs That Do Not Include A Health Bar:
Guardian, Shulker, Wither Boss, Ender Dragon, and, All Entities That are Not Accessible Via Spawn Egg (Boats, Armor Stands, etc.), Llama (Until they are damaged, as they have more than 20 hearts).
For Minecraft 1.11.2