Quote Originally Posted by chaika
it's an idiom, белая ворона. А вы не знаете о multitran?:

http://www.multitran.ru/c/m.exe?t=781608_2_1&ifp=1

a lot of meanings, pick the right one for your context. Maybe "odd bird" or "odd duck" might do.
Maybe "ugly duckling," black sheep," or "queer bird" would be better "rough translations?"