During the "Master of Horses" quest, I spoke with Dennet and unlocked the option to mount the Fereldan Forder. Instead of immediately doing so, I became distracted exploring the area. Eventually, still without mounting the horse, I traveled back to Haven. The Fereldan Forder was available in the stable at Haven, along with all my DLC mounts. When I returned to the Hinterlands (many gameplay hours later), the game froze the moment I approached Redcliffe Farms. I tried entering the area from several different directions, all to the same result. I don't have this issue anywhere else in the Hinterlands.
In the quick menu, the option to summon my mount is locked -- despite the fact that I have the mounts available for use in Haven. Obviously, the game is confused about what point I am in the questline, which is why I believe it's the cause of the freezing.
Unfortunately, I didn't keep any saves before my first visit to Redcliffe Farms, so this bug broke my game. I don't expect any fix that could salvage my playthrough, but I figured I should let tech support know for future investigation. Hope no one else gets as easily distracted as I do.
May 2016 - last edited May 2016
I also had this problem. Fixed it by going back to Haven to the stable and selecting Fereldan Forder as the mount. Then I could go back and do the mount quest without the game freezing. I believe the freeze is caused by the wrong mount spawning which the game does no expect.