You can now customize your Pets at costume shop.
There are two ways to go to your pet shop.
When you select any level, your game will provide you with pet suggestions. Click on the paw icon and you will be able to see your puppy, tokens and costume shop (T-shirt Icon)
You can also go to your Penelope prize shop and click on the item that has fashionable puppy and labelled as "Shop" which will take you to the pet shop.