Herobrine
Many people say that Herobrine is not an existing mob and the other people say that it is real. The true answer to this question is 'it is REAL'. It used to exist in the game, with infinite health, and did a lot of damage. He was banned from the game in the version 1.5.1 because no one could kill him and many said 'Remove it'. So it was banned from Minecraft. You can still have Herobrine by using a mod.