How to Upgrade/Unlock Treasures

Once you get a mythic to level 10 you unlock "The Altar". From the altar, you can spend Kitty Treats to get a random treasure for the altar. You can only have one treasure at a time. Every mythic also has their own unique "Exclusive Treasure". The Exclusive Treasure is the rarest treasure there is, but is always the best one, offering unique and powerful bonuses. Rocket Chu's exclusive gives you a 30% chance to avoid exploding when you fail his Overclock, letting you attempt to overclock again. It also reduces the number of attacks required to use his missile launcher skill by 20%. So it makes his biggest drawback much easier to deal with, while also increasing his damage.

When you hear people talking about treasures, and exclusives, this is what they're talking about.