This rule was introduced in Pet Parade,and Pet Patrol! and is specific to that game.
This is the cost (in loot) to bring the Pet into play. This is the only attribute which Pets have and Heroes do not.
A Pet’s summon cost is found on the front of its card in the bottom left hand corner. To pay it, the Hero must discard a number of loot cards equal to the listed cost. These loot cards may be discarded from those equipped to the Hero and from the backpack, in any combination. Treasure cards may also be discarded in this manner, in this instance a single treasure card is equal to three loot cards. If the cost is not paid the summon pet ability has no effect.