Technically you could, but you would have to make all new animations for it and you may end up having to reduce polies too (depending on the mesh.)
Nah, just rig it on the old horse skeleton, done stuff like http://i6.photobucket.com/albums/y233/Bahamut_ZERO/Icecrown/MorrowindScreenshot1.jpg
The real problem is that, the saddle workaround and overall feeling of riding kills, the expierence