This article is still developing...
The April 2014 issue of CoroCoro has unveiled that Raichu will be getting a Mega evolution. The leaked issue doesn't show much more than the top page, so it is unclear if any other new Mega Pokémon are coming with Mega Raichu or when said Pokémon will be available in game. Our source would like to stay anonymous for now. We are still waiting on translations for the page.
Update on Translations
Starting from the top rainbow text.
Giant Mouse Pokémon