In Val Royeaux there is a merchant on the top floor (accessible by fast travel) that sells a "Mysterious Box" for. After purchase, a golden nug will appear in the shop. If you interact with it, you will be given a new War table operation called The Big One? After completing the operation, all five nuggalope mounts will be available at the stables in Skyhold.
Once acquired, it can be found in the stables in Skyhold.