To apply a DLC fodder to change the pet appearance it needs to be done in your inventory BEFORE you place the pet onto the ground. I’m sorry it’s not after you put out.
So craft the fodder. Have fodder and adult animal in your inventory. Look in the list of your recipes in your inventory to locate the correct one for the pet type you wish to make. Craft and that will consume the fodder and adult pet will be changed to the DLC pet. Then you can place into the world.
This is the wiki page: