Why is Herobrine's name Herobrine

His name is herobrian because his real name is Brian but he fond a diamond in a strat cave and a creeper bloow him up his grave got struk with lightning and he tunned evil!

あなた could ask Mojang, but maybe they won't tell you. Write a letter to Mojang, and maybe they'll tell あなた why Herobrine's name is Herobrine!

The real Herobrine was a glitch in the Alpha stages of the game. Players would be walking around doing whatever, digging holes and building things (oh yes, I just went there XD), and minding their own business when they would suddenly and inexplicably begin to lose health. When the player died, it would say that they were "killed によって Herobrine". I'm not a game coder, I just play, but it sounds like this is a similar glitch to the HALO glitch where the player will suddenly be killed によって "the Guardians" for no reason. Whatever it is, it doesn't exist anymore.
This did escalate into the huge fandom it is today, which I personally am a part of, though not quite in the way you'd expect. But Herobrine has definitely become Minecraft's most famous unofficial mob boss ever, and a very interesting character.