Guys, right after that tweet, he credited Mo'Creatures developer(s) for helping with it. AKA the functionality won't be vanilla stuff. It was helped by someone experienced in specialized mobs specific to the users wants, someone who has heard complaints about their own riding system and tuned it to be good.
Then again, I will say he didn't specifically say it was the horse, and this could be a massive troll where it's actually like "We're adding back the human mob as an actual NPC" or something. In any case, Dinnerbone says he's working on some interesting feature that can't be expressed in a picture.