It is not possible to transfer your creature back from Creature Isle to Black and white. Technically, he's never "transferred" over from Black and White to Creature Isle either. He's just copied and then modified for Creature Isle to let him have the extra abilities you can teach him there.
Now if your problem is that you tried copying your creature's file back into Black and White without backing up the original file, then yes you are in trouble. You won't be able to play with him at all in Black and White now. You'll just have to start with a new creature there I'm afraid.