There must be a glitch in the game or else the Pokemon must have been obtained from some cheat (the ditto or the other Pokemon with which you had mated your Pokemon to get zorua ) .For my own zorua had learnt the move night slash at level 30.I can prove that my zorua had learnt the move on lv 30 for I have it with me and if you request i'll make a video or send you pics after my exams april 4 -18
source ::: personal experience