There were three Christmas skins added in December 2015, Gingerbread, Santa, and Evil Elf. Unfortunately, these skins are no longer available in the Agar.io store. The Christmas skins were updated in December 2016, with three new skins: Elf Helper, Santa Claus, and Reindeer. The previous skins did not return in the 2016 update, however.
“The Evil Elf is a Christmas skin and can be bought from the Agar.io Store for 4250 coins. It is...Evil Elf