I don't know idle he's real, but once I thought I saw him in my world in the desert a way off. Then this one time where my house stared to burn and I went outside and saw him under a tree in the winter forest. When I went to investigate the spot, he was gone and that's the last time I saw him.
Basically, some guy (prob creepypasta) created Herobrine. Then stories spread 'round the Minecraft community, and then some coder created a mod that had Herobrine in it. The legend evolved from there, and Herobrine became known to 98% of the Minecraft community. If you'll notice, at every MAJOR game update, (such as 1.5, 1.6, 1.7, 1.8... NOT like 1.7.2, 1.7.4, 1.7.9, etc.) It says "Herobrine removed." Notch did this as a joke. Herobrine never truly existed, but his legend still runs in the Minecraft community. And that explains the story of Herobrine. And.. I'm out.
Think about how easy it would be to fake herobrine. Go to a private server with a friend, have a friend change his skin, turn off names and voila! Spooky herobrine sighting.
Creepypasta created it. Along with other fake things like Slender Man, Eyeless Jack, The Rake, Sonic.exe, Ben Drowned, and many more.
Wait, Slenderman is creepy pasta? Wow, I thought it was inspired from a real event. Also, I googled Sonic.exe. WHAT IN THE WORLD IS IT?! THE RESULTS WERE CREEPY AS HELL!
