Did you complete the horsemaster quest? If you did, you have stables at your camp (in Haven next to the place where you can upgrade your weapons and armor, in Skyhold next to the shops) Just click the stables, select the horse (the screen will go black for a moment) Normally the horse will turn up in the stables. When it does, go back to the hinterlands and the horse should be there. I've had the same problem and it worked for me, so I hope it will help you too. If it doesn't work on the first try, don't worry...just try it again a bit later in the game. personally I had one character where it worked from the first try and another where I had to try a couple of times. If the horse doesn't show up after the black screen, you can also try to select other mounts until one appears in the stables. I have not tried this one, but I've read somewhere that it works so...