Xiahou Dun, the One-Eyed

If a turn has multiple combat phases, the ability can only be activated before the beginning of the declare attackers step of the first combat phase in that turn.

Despite the similarities between horsemanship and flying, horsemanship doesn't interact with flying or reach.

You can't activate Xiahou Dun's ability to return Xiahou Dun itself to your hand. You must choose a target before you sacrifice Xiahou Dun, so it's not a black card in your graveyard yet.