The Battle Nug is a type of Nuggalope that can be used as a mount. To do this you must travel to Val Royeaux, and look for a vendor named Deraboam who is in a small store which has a blue door. When you interact with the chest you will find 'The Mystery Box' unique item which costs 10,000 coin. Buy it to unlock 'The Big One?' mission on the global war board and then send one of your advisors to complete it. When they return from the mission they will have the giant War Nug mount.