I already did the Master of horses quest, so I've the option to change my mount on Haven when I go to Dennet location, but all of them are blocked. How do I get new mounts?
According the IGN:
Mounts are rideable creatures in Dragon Age: Inquisition that can either be found, purchased, or rewarded by completing certain quests. There are plenty of mounts, including exotic mounts, that can be found in different places of the world. Other mounts can be purchased once you have convinced Horsemaster Dennet to join the Inquisition, with more mounts available to buy over time.