A few hours into your Horizon Zero Dawn journey you will unlock the ability to mount enemies. Although a quick tutorial explains how to perform this maneuver, many players have found difficulty in figuring out how to do it.
After unlocking mounting, all you need to do is sneak up to a rideable enemy undetected. When behind the enemy you will see a button prompt for override. This button to begin mounting the enemy.
Various enemies can be mounted for different periods of time. Take advantage of this to travel between areas, or even quickly transition to a secondary enemy in battle.