A Frosty Creature
A mysterious Pokémon has recently been seen in Emera Town. It's said to cover everything in its surroundings in frozen ice.
The Frosty Creature is a Kyurem. This event started the 4th of December and ended on the 19th of March 2014.
Kyurem was then once again obtainable 2015 (6/Jan to 10/Feb), 2016 (13/Jan to 1/Mar), 2017 (5/Jan to 13/Feb) and 2018 (1/Jan to 8/Feb).
Kyurem can be obtained by interacting with Eggs & Pokemon. An icon of Kyurem will randomly appear on your userbar while interacting. If you see the icon, you can click on it to get the egg, provided you have a free space in your party to obtain the egg, but if you don't, you always have time to make space. Note that it is not required to have a free slot in the party for it to appear in the userbar.
After accepting the creature's gift, you will have a Kyurem Egg in your party.