The only way to get Zorua is through an event that passed.
Short answer: Yes, to complete the Pokédex, you need Zorua and Zoroark. Sources:
According to Reddit.com, "They are part of the unova pokedex meaning if you cant get them in game then its impossible to complete the pokedex." (https://www.reddit.com/r/pokemon/comments/4xnt15/zorua_and_zoroark_in_pokemon_black_and_white/)
According to Psypoke.com: "you need both of them to complete the National Pokedex." ( http://www.psypokes.com/forums/viewtopic.php?p=474290)
That said, here are some ways to obtain them:
According to Gamesradar+.com: "Zoroark is obtainable in B&W via an event in Lostlorn Forest if you've obtained any of the shiny legendary beasts from the GameStop/Wi-Fi download event. Zorua is also obtainable via an event in Castelia City if you have the Celebi from the 2011 US mall tour or the B&W pre-launch GameStop event. Don't worry too much though if you miss out on the events – Zorua and Zoroark aren't legendaries, so you should have no problem finding a kind person to breed one for you and trade." (https://www.gamesradar.com/pokemon-black-and-white-pokedex-zorua-zoroark/)
If you can't obtain them this way, then try out diehardgamefan's suggestions. (http://diehardgamefan.com/2011/03/08/pokemon-black-and-white-obtaining-zorua-and-zoroark/)
I hope this help Sqealbert!
