Feebas, if you are wondering where to get it there are 2 options, 1.) Go to Mount Cornet and surf, it only has 4 tiles it will appear in each day. 2.) Fight a Fisherman in Route 222, it is facing the sea and is the second from the right.
You can never trade Pokemon from Black to Platinum, because Generation V Pokemon do not appear in Platinum or any other game before Black and White. However, you an transfer Pokemon from Platinum to Black after you beat the game.