Do you have to send over a legendary dog to get zorua in black and white?

Yes, to obtain Zorua in Pokémon Black and White, you need to have a special event Pokémon, specifically a legendary dog (Raikou, Entei, or Suicune) from Pokémon HeartGold or SoulSilver. When you interact with the man in the Pokémon Center in the town of Driftveil, if you have one of these legendary dogs in your party, he will give you a Zorua. This event was designed to encourage trading and special encounters between games.