After using the Memory Link with a copy of the first Pokemon Black and White, you will learn how N released all of his former Pokemon. In Pokemon Black 2 and White 2, you will be able to capture these Pokemon in the wild.
All of N's former Pokemon will give off a green shine when you first encounter them, similar to Shiny Pokemon, and similar to the move Embargo.
Each Pokemon will always be at the same level, have the same Ability, Nature and Move Sets along with a constant IV (individual value) spread of 30 in every stat. The only Pokemon that is different from normal Wild Pokemon is the Darmanitan with its Hidden Ability Zen Mode.
After capturing them, it will say "It seems like the caught [Pokemon's Name] has a different OT [Original Trainer]!" Capture them to see their Original Trainer is N and their ID Number is 00002. Seeing as how they were formerly N's Pokemon, you won't be able to nickname any of them.
All of N's Pokemon can only be caught once, but can be rebattled if they were knocked out or if you fled from the battle.