Depending on its type, you may be able to evolve your pet. Evolution is only possible when the pet is summoned. Also, since the process transfers the experience value, evolving a high level pet might actually result in a decrease in the pet's level.
Any items in the pet's inventory will be transferred to the owner's inventory.
Which pet do you want to evolve?
Evolve a Wolf into a Great Wolf.
Evolve a Great Wolf into Fenrir.
Evolve a Baby Buffalo into an Improved Baby Buffalo (for Warriors).
Evolve a Baby Kookaburra into an Improved Baby Kookaburra (for Mystics).
Evolve a Baby Cougar into an Improved Baby Cougar (for mixed professions).