When you first ente Driftveil City and help old Team Plasma out, you will get given a special Pokémon, N's Zorua. Normally, this would just be it, but the sage informs you, possibly only if you used Memory Link with Pokémon Black & White, that there are more of N's Pokémon in the wild

Using Memory Link, you get a flash of when N started releasing his Pokémon within Chargestone Cave. This will activate the event and you'll be able to find all the Pokémon that N had on his team before the end of the game of Pokémon Black & White. These Pokémon are found in the wild, and give you a note as battle starts, indicating that they may not be wild. It won't let you rename them.

Each of N's Pokémon have the OT of N and ID of 00002 so it's very easy to work out who they are. They also have a note in their status screen saying they used to be owned by a powerful trainer.

You will notice these in battle when the battle begins. They will shine, akin to Shiny Pokémon. It will also prevent you from nicknaming them once captured.