Version-exclusive Pokémon in Black & White
Like all the paired games in the Pokémon series, Black & White each include Pokémon that cannot be obtained in the alternate version. To obtain Pokémon missing from your version you need to trade with someone who has the opposite version.
Furthermore, Pokémon White contains an exclusive route, the White Forest, which contains a large batch of Pokémon. The equivalent in Pokémon Black is the Black City, which has more trainers but no wild Pokémon. Note: Cottonee and Petilil are only exclusive in the wild to Black and White respectively. The
unobtainable one is available in each via an in-game trade.