Some loot cards are parts of Legendary Sets that have been separated, and reuniting them unlocks their full power. An animal icon is noted at the bottom of each item in a set along with the bonus effect they grant when multiple items in the set are equipped together. There is no set bonus for having only one item of a set, but having even two unlocks the first level of the effect. Each extra piece increases the bonus.
For example, having 2 items in the Lion set heals yourself +1 each time you make a Melee Attack. Equipping all four set items unlocks the highest bonus. With the complete Lion set: heal +3 for each Melee Attack.