Pet attributes consist of:
Base Damage: Pet can shoot out Fire-ball which deal AOE damage to monster. The damage of the fireball is based on this damage and the Level of passive Fire-ball skill of the Pet.
Fire-rate: The rate which the Pet can shoot out. This is equivalent to how many Fire-ball/Ice-ball in one second.
Energy: Affect on utilization of the Pet. When Pet energy is zero. It need to rest and can't be used for any action.
Pet size comparison
From the strongest one is the Mythical Red Dragon to a Common Red Parrot.