The official Japanese Pokémon Website has announced details of a Pokémon Event distribution which will take place at the Pokémon Lab: You do it! You discover! (ポケモン研究所 ～キミにもできる！新たな発見～) special exhibition. Those that participate in the professor's mission in Lab Room 1 (第1研究室) at the Pokémon Laboratories will be given their favorite Pokémon out of Fushigidane, Hitokage and Zenigame. At the end of that mission, players will receive a serial code that can be redeemed in Pocket Monsters Omega Ruby or Alpha Sapphire or Pocket Monsters X or Y based on that Pokémon selection. Each of these starter Pokémon is holding a Mega Evolution Stone.

Event Leaflet Scan

Event Pokémon Distribution Details

Exhibit Details

Exhibition Days: Wednesday July 8th, 2015 until Monday October 12th, 2015
Serial Code Redemption Period: July 8th, 2015 until November 30th, 2015

Source: http://www.pokemon.co.jp/info/2015/06/150605_e01.html