It seems like I read (forgive me, I don't remember where) that the spectral horse in Dawnguard was a summon. I quick google search hasn't told me much.

Had it been confirmed or have I been imagining things?

Yes you will be able to summon the horse.

this is confirmed. it will be a power like the summon spectral assassin and only usable once a day. if i find the link confirming that i will post it later.... maybe