From the last update of the game it will be possible for you to customize your pet; that way you can give it a new look and add new accessories.
To do this, go to your pet's section by tapping on it on the main screen of the game.
Once you do, you will see a button represented by a hat in the upper right corner of the screen, and if you access it you will be able to buy different hats and appearances for your pet
To unlock these items, you will need to buy cards using credits; each item will need a certain amount of cards to be unlocked and used.
Every time you buy a card, you will see which item it contained and, in case of obtaining all the necessary cards to unlock an item, it will appear as available to equip.