Regional mounts. Every city-state has at least one monster unique to that region that can be tamed and used as a mount, the individual requirements vary but you need to be at least level 30 to even try unlocking them. Further, you do not need to be aligned with that city-state or a member of their particular Grand Companies but you do need a considerable amount of fame to start the quest. In basic parlance, you must be at the point in the main-scenario where the Grand Companies are vying for you to join them. You can then start the quest at any time.