If you haven't already gotten it or weren't aware of it in the first place, now's a great time for you to score a super-cool Shiny Beldum holding a Metagrossite from Mystery Gift!
That's right, until January 14th, 2015, you can receive a Shiny Beldum by selecting Mystery Gift from the main menu of Pokémon Omega Ruby or Pokémon Alpha Sapphire, and then choosing to receive the gift "via Internet". (Note: this gift is not available in Pokémon X or Pokémon Y.)
Shiny Pokémon are normally super-rare, so this is your chance to have one of them for yourself. Not only that, but since it holds the Metagrossite, after it evolves into Metang at level 20 and then Metagross at level 45, you can Mega Evolve it into a Shiny Mega Metagross by triggering Mega Evolution in battle while it holds the Metagrossite!