Keldeo cannot be caught in-game in Pokémon Black and Pokémon White because it's an event-only distribution Pokémon.
Currently as of January 22, 2013, Keldeo cannot be received by event in Pokémon Black and Pokémon White; however, you can either opt to trade for one or you can opt to choose a cheating device in order to acquire one.
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Keldeo is event-only pokemon. This means that Nintendo will distribute Keldeo for players to catch. The only other way is to cheat with Action replay, gameshark etc.
It will be in Special Events in the future. Hopefully, soon.
You can't catch it in Black, but you can catch it in White. In the White Forest.
you have to get in Pokemon white
not catch but by Pokemon transfer or the Dreamworld