This page has the details about the Shiny Beldum distribution that took place after the release of Pokémon Omega Ruby and Alpha Sapphire.

Steven's Beldum

From November 21, 2014 through January 14, 2015, you could download a Shiny Beldum in Pokémon Omega Ruby and Alpha Sapphire. The Beldum was holding Metagrossite, which is the Mega Evolution stone that allows Metagross to Mega Evolve into Mega Metagross. The Beldum knew Hold Back, Iron Head, Zen Headbutt, and Iron Defense. The Beldum's original trainer is Steven.

How to Download

The distribution has ended, so it is no longer possible to download this pokémon.