I know that mounts are mentioned in the rules book, in passing, when discussing the stats for a Hare. No mechanics are given in the core book. Have errata/clarifications/add-ons been produced? Either 1st party or 3rd party in origin. Would it be hard to add in?
I have the second edition box set, and those rules are in the expansion. I also managed to find a pdf version that someone had posted online (first edition version). The short answer to the question, though, is that you primarily use the mount's nature for rolls. And being mounted limits the kinds of weapons that you can use while mounted (spears, bows, etc.)