So let's say horses won't be the only way of traveling faster, what mount would you like to have? And mind yo,u no dragon-riding bullfeces...
I just had this hillarious idea of dovahkiin on skies, i mean since it's snow everywhere wouldn't it be a good way to travel? No but seriously, mammoths perhaps? Or on the Shoulders Of Giants^^
Taking into account that you have to get down from your mount anytime you want to fight, the mount becomes simply an aesthetical accesory. Thus, I'd vote for anything to be mountable by default (in the engine and for modding purposes) and then leaving it active just for horses, camels, big wolves, etc.
AWWWWWWWWWWWWWWWWWWWWWW YEAAAAAAAAAAAAAAAAAAAAAAAAAAA But seriously, I'd like to run into town, jump on some guy's back, and scream "PIGGY BACK RIDE!!!"