Herobrine:
Mysterious, unknown figure that looks exactly like you but without pupils, always appears at a distance where you can just see him enough to know it’s him, and he is not supposed to be there but somehow he is. Think about a carbon copy of you without any pupls/iris. Those would be perfectly white/bloodshot eyes, but you. CREEPY!!!

Slenderman:
Another mysterious, unknown entity. The slenderman defines fear in a nutshell. You are no match for him, all you have to do it run. If you happen to see him, you’d better go the other way. If you look for too long, he quickly teleports to you and makes a pants-crapping scary sound, and who knows what happens next? As you go on he gets faster and faster, and since it’s best not to look at him, there’s no way to know just how close he is to being able to get you. And again, who knows what happens when he gets you? Faceless, 9 ft. tall man with black menacing tentacles coming out of his back.

I think that sort of beats at least bowser. 😛