I don't think Bethesda can leave mounted horses out at this point.
1. They are already in, but Bethesda is getting the mount mechanics together. If they can't get the mechanics together in 7 months, then there are deeper problems then horses. I don't think the game designer well say that they are working on it only to take it out later. (once you let the cat out of the bag, its out)
2. Its 200 years after Oblivion and the Imperials had mounted horses. So are the Nords dumb and can't figure it out 200 years later? Nords will have mounted horses too.