There are lots of cool pets in Portal Knights, and any of these can be attached to your character. When you do this, the pet will follow you around when you play the game, giving you a cute companion.
The pets don't do any damage or help you out in any way, so feel free to use whichever pet you think looks the best alongside your character.
In order to have a pet follow you, first make sure it is in your inventory and not in a chest somewhere. Now head to the equipment screen where you change your gear, and look to the bottom left corner. There is a small, kind of hidden pet slot there. Just click on this and you get to choose between all the pets in your inventory. Equip the pet you want to follow you, and it will be moved from your inventory to the pet equipment slot, where it will stay until you either remove it or replace it with another pet.