Pets can be upgraded to Legendary by reaching max Pet Expertise and spending Pet Growth Tokens, which can be earned by exchanging Jam Cookies. Once a Pet has been upgraded to Legendary, several features are unlocked. Legendary Pets are granted a (random) powerful skill, and at random intermittent intervals, one of 3 effects granted by the skill will be applied to you while in combat. There are 5 Pet Skills which can be randomly unlocked, or switched between:
A Pet Skill can be changed at a Pet Management NPC in exchange for 20 Crystals. You can select which skill you want, and it can be changed again whenever you’d like.
Legendary Pets can also have their appearance changed to match that of another unlocked Pet. Changing a Pet’s looks requires a Pet Customization Potion obtained from the Cookie Vendor, but it only needs to be used once per Pet. Afterwards, that Pet can have their appearance changed any number of times. Finally, obtaining a Legendary Pet unlocks access to purchase one more slot in the Pet Inventory for more storage space.