Advanced Pet Training Potion
Tamers of the very highest skill can now create a potion that will raise a pet's max level by one. It is a difficult and expensive item to craft but it can be quite useful on all of your favorite pets.
You'll find the item under the "Misc" section of the pet brush crafting tool.
It will not work on bio pets since they don't use levels, and it will not take a pet past level 30.