From Bulbanews, your community Pokémon newspaper.
Distributed nationwide at Pokémon Centers and Stores
To celebrate the opening of the new Pokémon Center Hiroshima, Magikarp and Shiny Gyarados will be distributed at Pokémon Centers and Pokémon Stores across Japan. These distributions can be obtained in both Pokémon X and Y as well as Pokémon Omega Ruby and Alpha Sapphire. Both distributions run from June 26 to August 31, 2015 at Pokémon Centers and Pokémon Stores throughout Japan.
At these stores, players can obtain Magikarp from the "Magikarp Fishing Spot". Magikarp can be obtained multiple times in a single game. Unlike most events, it can be traded through Wonder Trade and the GTS. This Magikarp has a very small chance of being Shiny (possibly the regular Shiny rate).
Shiny Gyarados can only be obtained once per game, and cannot be traded through Wonder Trade or the GTS. Red Gyarados is one of the mascots of the new Pokémon Center Hiroshima.