You can only get the special Zoroark if you transfer the " Event version " Legendary beasts.
There was an event a while back ( Little bit before B/W was released ) that gave you a Shiny Raikou, Shiny Entei, and Shiny Suicune on your Pokemon Heart Gold and Soul Silver game.
They also had a few other key things about them.
Anyway, you can only obtain Zoroark that way if you traded the event ones, which according to your comment, you don't have.
So, you're out of luck. Try looking for a Zoroark on the GTS
( N has one in the final battle, so you should have it on your pokedex )