Mystery Stockings contain Donuts or a Reindeer and cost 50 Santa Coins. There was an update on January 2, 2013 installing the Mystery Stockings and the dialogue following into the game, but they didn't appear until January 3, 2013. The stockings were introduced to allow the players to convert their excess Santa Coins to permanent items, which was opposed to the experience after the Halloween content update, when the surplus of Treats just vanished.
|“||Use surplus Santa Coins to buy Mystery Stockings before the Holiday Event ends on January 10th.||”|
— In Game Message