Christmas Tier 4 Egg was added in Update 14. This egg costs 8,000,000,000 Snow Coins.
There are 4 Christmas pets that can be obtained by opening this egg during the Christmas event.
Note: Rarity is based off of the pop-up menu when going near an egg/present. The "??" Rarity is also known as the "Godly" Rarity, since pets/hats in this category are much more challenging to obtain.