RDR horses + Mount&Blade mounted combat = Sorted.
I'd be quite happy with having a horse similiar to the Assassin Creed games though, to be honest. Don't need anything overly fancy for Skyrim, maybe for the next TES game where there are large cavalry armies and stuff.
Aggro from Shadow of the Colossus is the way horses should be in videogames.
RDR and Assassin's Creed are mere derivatives.
Mount and Blade has all of these. Therefore, Skyrim should copy it.
Seriously, Mount and Blade has a much closer setting and combat to Skyrim than RDR or Assassin's Creed.
I haven't played any of these games apart from Shadow Of the Colossus, in that the landscape is quite open, almost like a giant flat plain with wide corridors and it would seem like it was created with Aggro in mind. Console third person kind of openess.
Now like i said i haven't played RDR or Mount and Blade, but from the screens and videos i've seen of them, they seem similar to Shadow in that they were designed specifically for horses.
RDR = Open Mojave/Mexican deserts and pairie plains with a few trees
Mount & Blade = Open English/European countryside and a few forests
Now just imagine using a horse in what we've seen of the terrain of Skyrim, trudging your horse up a snowy mountain with a Dragon attacking you hmmmmm