How to Catch Zoroark
If you use the Relocator to transfer Entei, Suicune, or Raikou to your game, then you can catch Zoroark. First, you must get to Nimbasa City in the game. Once there, head northeast to Route 16, and check the northern part of that area to find the entrance to Lostlorn Forest.
In the northeast part of the forest, you will find a camper parked there. If you bring the shiny Entei, Suicune or Raikou with you that you transferred with the Relocator, then a woman will dash out of the RV and growl at you, and then a pokémon battle will begin.
If you bring Entei, then the pokémon will appear to be Suicune. If you bring Raikou, it will appear to be Entei. If you bring Suicune, it will appear to be Raikou. However, once you hit the pokémon once, it will transform into Zoroark, revealing that the other one was just an illusion.
Tips for Catching Zoroark
Although Zoroark is somewhat rare, its catch rate of 45 is not as low as legendary pokémon like Victini.
False Swipe will help. If you have seen at least 30 pokémon according to your Pokédex (you most likely have by now), then you can get the False Swipe TM from Professor Juniper in Nuvema Town. Leavanny is one of the few pokémon that you can access at this point in the game that can learn False Swipe from the TM. If you don't mind waiting until you reach Route 6, then you can catch a Karrablast there and teach it False Swipe.
Using False Swipe and a pokémon who can inflict sleep, bring Zoroark's HP down to 1 and inflict sleep on it. Then, if you have a Dusk Ball and you started the battle between 8:00AM and 3:59AM, you will have a 100% chance of catching Zoroark. You can also use a Fast Ball and get a 100% chance, or use a Timer Ball after 30 turns.
Otherwise, if you use an Ultra Ball on the sleeping/frozen, 1HP Zoroark, you get about a 69% chance of catching Zoroark.
If you use a Nest Ball on the sleeping/frozen, 1HP Zoroark, you have about a 53% chance of catching it.