Like all the paired games in the Pokemon series, Black 2 & White 2 each include Pokemon that cannot be obtained in the alternate version. To obtain Pokemon missing from your version you need to trade with someone who has the opposite version. Due to the Unova Pokedex being expanded in Black/White 2, there are a few more exclusive Pokemon here.
Note that a few Pokemon listed here are available via other methods: Cottonee/Petilil are available via an in-game trade, and Regice/Registeel can be obtained using the Unova Link feature.