A new mount like a new model, or a new horse in the CK? I assume the latter you already know.
I know of this tutorial http://skyrim.nexusmods.com/mods/13865. Never tried that sort of thing, so I have no clue how/if it works. I think it deals with sabrecats and bears and whatnot, but you have high skill in 3D modeling so I guess you can adapt.