Have atleast one of the Incarnate forms in your party. Find them in Dynamax Adventures, Trade, or Transfer.
Go to the Bargain Shop in Stow-On-Side. A man will give you the Reveal Glass if you have one of the Incarnate forms in your party.
Use the Reveal Glass on one of your Incarnate Forms, and it will revert to it’s Therian Form
Hope this helps!