Mounts in Horizon Zero Dawn are covered on this page. Aloy can use her Override ability to hack enemies and convert them into mounts that can be ridden across the world, dramatically improving her speed. The machines that can be converted into Mounts are listed here.
When dismounted, Aloy can whistle for her mount to call it to her location. If Aloy has unlocked the skill "Call Mount +" she can call a mount even if she has not overridden a machine prior.