Some people say he was a mob gone wrong. I believe them. One story states that Mojang tried to delete him, but he just got stronger. Whatever the story, he's real. Notch says minecraft and Mojang have nothing to do with herobrine. If this is true, why do some minecraft websites that Mojang created have a picture of herobrine? And why does every single update since 1.5.2 say -Removed Herobrine? It may be a prank, but if Mojang really had nothing to do with Herobrine, they wouldn't even do a prank.
And for all you people who don't believe in Herobrine, 'Seeing isn't believing.' Just because you haven't seen him, doesn't mean he's not real. I've never seen him, but I'm a true believer. I've been one ever since I heard of him, when I was about 8 years old. That was a while ago. And even if he's not real, that doesn't mean you have to be mean about it! People can believe whatever they want to believe. So keep out of this!