Thief is another move many Dark (and other) Pokemon can get, though Zoroark will probably only be learning this by TM.
I agree that Night Slash would be a good move, just looking at Z'Ark's claws. Dark Pulse would be more likely if it has good Special Attack (which I'm hoping it does).
It'll likely have Dark Pulse just for the Special coverage, regardless of it's Sp. Atk score. Whoever programs the moves and other components for each Pokemon doesn't seem to worry about giving Pokemon the best movesets they could have (see Flareon and it's lack of Flare Blitz, for example). I could see Z'ark with a good Sp. Atk score, but I'd prefer for it to be better physically.