Is it me, or does this mini quest play out as if both ghosts are saved from the Soul Cairn at the end? The owner thanks the player then disappears, whereas all other ghost remain permanently. The same for the horse. The owner talks as if you saved Arvak as well. Come to think of it, Serana said that animals don't go to the Soul Cairn, even after being soul trapped. If so, why is Arvak there in the first place?

I think Arvak is awesome, he just seems slower to me than other horses, that or maybe it seems like he doesn't have as much stamina. I have no idea why he would be there.