The internet never misses anything, and a Reddit user happened to spot a listing in the Xbox Live store for the upcoming Halo 5: Guardians beta. It seems to be a pretty big beta with an installed size of 10.27 gigabyte data.
It will be available for the people who bought Halo: The Master Chief Collection, but there will of course be other ways to enter it as well. The beta will start December 29th and last until January 18th. Some of the stuff you unlock in the beta will later be made available in the retail game when it's released second half of 2015.