The mantra of the pokémon trainer has always been to catch them all. This is not always an easy task. Thankfully, every so often, Nintendo tosses us a bone. From now until the end of Halloween you can get a free Supersized Pumpkaboo if you have Pokémon X / Y. Just check for a gift online before launching your game.
Once recieved you will be able to pick up your new friend on any Poké Center you come across. Enjoy!