I have SoulSilver and I caught Mewtwo in Kanto.Then I used Poke Transfer and send it to White 2,which of course it is a Unova game.I checked Mewtwo's info and it says "Johto".Why Johto since I caught it in Kanto ?
When a Pokémon is transferred, its location becomes simply the name of the region it was caught rather than the exact location it was caught. A Pokémon caught in HeartGold or SoulSilver will have its location listed as Johto, regardless of if it was caught in Kanto, the Sinjoh Ruins, or the Pokéwalker. A Pokémon originally caught in a Generation III game will have the text "after a long travel through time" added to the end of its location. In addition, the level obtained at is changed to the level arrived at, and the date obtained becomes the date it was transferred to the Generation V game.
SS and HG are Johto based games as they originated from Gold and Silver which only had Johto as the region. Source: Experiance and Bulbapedia