OK, look, this is not hard to edit using Preview on a mac, or even to re-texture a door or something else to do this, and there's even mods to add him now! This is obviously fake!

I sometimes go on servers with a herobrine skin on though too, you don't think someone else could? The name tag wouldn't show up because the herobrine here is at the top of the screen. Very convenient, since you said you "just saw him", considering you were looking at the ground. Why would you be looking down anyways? Obviously not for mining, because that's clearly some sort of road.

Even more

There is no skin file for him in minecraft.jar

Mobs are invisible without a skin. He doesn't exist.