In Val Royeaux there is a merchant on the top floor (accesible 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 A 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.