legendaries

"An awesome opportunity is on the way! Starting on May 8th, 2013, get the Mythical Pokémon Deoxys via Nintendo Wi-Fi Connection. This Lv. 100 Deoxys knows a quartet of amazing moves and is ready to lead your Pokémon party to victory! Deoxys will be available for your Pokémon Black Version 2, Pokémon White Version 2, Pokémon Black Version, or Pokémon White Version game only until May 31st, so don't miss out!"

One month you can get a Legendary Pokemon, If you miss it, you have to try to get it from others. All Pokemon should be available in one game to catch.