It's a quest you have to start. First, go to Orcish Byway. It's a town in the human realm.
There is a girl highlighted by a golden beam at all times. Talk with her (named
Archeologist's sister) to begin the Special Mount quest. You have to go talk to her friend
who tells you that you need a map. Go get the map and return to her then take a boat up to
this little island where you talk to another guy who tells you that you have to prove
yourself in battle before you can buy your mount. You travel to the other end of the
island and fight groups of increasing difficulty three times, returning to the merchant
each time. Then you will be allowed to buy your mount.