I just thought about it and it should be that when you walk "inside" you automatically dismount. You wouldn't walk into the tombs with three people riding elephants, and you wouldn't walk into a shop trotting along on your horse.
Just a thought.
Automatic dismount
What are all these ideas about le frosty? What would the game play gain from that? What would we as players gain from it, except that fact that we would be extremely annoyed about getting constantly dismounted? Besides, what's wrong with walking into a tomb while riding an elephant? 
This makes me wonder when we'll have the first porcelain-shop in FL, where you'll pay the major bill if you go in while mounted... 
Of course, bill being multiplied with the size of your mount.
I dunno, I guess I just thought it 'made sense'.
It makes sense in a strict RP sense... but as a mechanic it would be fairly annoying. Having to type mount all those extra times would just make me want to punch a shopkeeper in the teeth.