in black and white versions, is keldeo a real pokemon and if so how do u get it?

Yeah, Keldeo is a real Pokemon. Unfortunately, あなた can't get it right now. It's an event Pokemon.

Here is it's page on Serebii.net link

Keldeo does exist. It is the fourth Pokemon of the Musketeer Trio (which makes no sense, so I call it the Musketeer Quartet). It is currently not available anywhere, I believe. There was a rumor floating around that there was to be a Keldeo event sometime between July and September, but it has yet to be proven.