Horse combat isn't part of the DLC. It's being added for free in the 1.6 patch.
I'd personally prefer werewolf stuff to be added in something that has more of a focus on werewolves, so it can be done with the attention and focus it deserves, and not have another repeat of where they feel tacked on and incomplete. Dawnguard will take less than a year to be developed, and it has an obvious emphasis on vampires and possibly some sub-plot involving the Snow Prince, so anything not related to that won't have as much care put towards it.
1.6 went into beta just before E3...
I'm speculating they are planning a release of dawnguard in the weeks following E3, with 1.6 riding parallel.
Horse combat was probably something they were going to have in the DLC (I mean there is a whole scene showing nothing but horse combat, why would they show that if they didn't want it related to the DLC?)
and just ended up being something they needed to test for bugs before release.
And yes I do realize we are getting it for free. But it seems pretty obvious to me that it supposed to go alongside dawnguard.
Besides, can you not see the other game jam stuff in the video? That is the point I was attempting to make: Gamejam
again I am not saying anything bad against Dawnguard, I just don't understand why the second someone asks if there will be werewolf content everybody jumps on him like "No! Shut up and wait your turn!"
if the developers want there to be a werewolf quest/perk tree in dawnguard then what is wrong with that? It's not going to prevent later werewolf expansions and its not going to affect the main story of dawnguard.