During December and until January 11.th 2017 you can get a free Muchlax with the Z move Snorlium Z for free as a Mystery Gift.
Start by booting the game and go to the main game menu where you can load your save. Click on the Mystery Gift menu, and click on Receive Gift. Make sure you are connected to the internet before doing this. Once you click on receive gift, you will need to wait between 60 and 120 seconds while the game downloads the Munchlax, and once it's finished you will find it in your party, or in a Pokébox.