Start/End Dates	2014-08-13 through 2014-09-17
Country	United States

Details

Starting on August 13th, 2014, players could receive a Heracross for their Pokémon X game or Pinsir for their Pokémon Y game. Initially these Pokémon did not have any held items but from August 18th, 2014 until August 31st, 2014, players could go to participating GameStop stores in the United States to get Mega Stones for the Pokémon to hold!

When players entered the code into their game, they received the matching Mega Stone: Heracronite in Pokémon X or Pinsirite in Pokémon Y. GameStop was also distributing a special preorder bonus poster for those that preorder a copy of Pokémon Omega Ruby or Pokémon Alpha Sapphire.

Heracross and Pinsir Distribution Instructions

You can receive Heracross or Pinsir via the Internet directly into your game. Follow these steps to receive the Pokémon!

1. Connect your system in the Nintendo 3DS family to the Internet.
2. Select MYSTERY GIFT on the main menu.
3. Select RECEIVE GIFT, then YES.
4. Select GET VIA INTERNET, then YES.
5. Watch as you receive Heracross (Pokémon X) or Pinsir (Pokémon Y)!
6. Speak to the delivery girl in any Pokémon Center to add Heracross or Pinsir to your party.
7. Be sure to save your game!

Source: http://www.pokemon.com/us/pokemon-news/unleash-the-strength-of-two-tough-pokemon/