So I was riding along in the rift when a headless horseman rode by he was a ghost npc named headless horseman is this added in dawnguard or have I just never seen him till now? Anyone else seen him?

It's not new. I've seen him before Dawnguard was announced.

Its part of the vanilla game too.

No, he's not new. Been a part of the game since launch.

o ok my bad it's just in all my wanderings I'd never seen him is he tied to a quest or anything or just a random easter egg.

I still havnet found him in over 300 hrs...

Random easter egg.

too bad

Random easter egg. Though if you follow him, he'll lead you to his body's resting place. I forget the name of it, but it's right next to the dungeon you have to clear with vilkas (or farkus) for the companions questline. The one you're sent after a wuuthrad fragment and you meet the silver hand for the first time.

He takes you to a graveyard with a master-lock chest if you follow him. Other then that, completely pointless.
Farkas

ha same here and im always running around whiterun at night

Been a while since I did that questline, couldn't remember who you were sent with...or even how to spell his name. xD