These are non-functional pets that follow your character around.
To use it, just right-click on the inventory item. Right-click on the inventory item again to remove your mini-pet from view. You can only have one deployed at a time.
Pre-Order Bonus Pack Pets Edit
If you pre-ordered the game there is a Rewards Verification Officer in Alia Das who will give you a mission to receive a pet. You may need to log off and log back in after your first session in order to be have the quest activated. This will give you a Pre-order Companion Token in your bag to be able to turn in to the officer to receive your pet.
If you have the limited collector's edition, the boo bot will automatically appear in your inventory.