Mounts unavailable at Skyhold.

Before I go and slay Corypheus during this playthrough, I am wanting to complete all collections within the game and sync them through the Golden Nug statue.
I’ve completed every available war table operation, and so far I haven’t been able to find every single mount to be added to the stables at Skyhold. 
Why am I missing four exotic mounts in the selection at the stables? 
Help would be appreciated if I’m told how to find and collect the missing mounts. 

Edit: I’m playing on Xbox.