^ I personally think Rukario looks kinda cool...*Gets slapped*
Anyways, I'm not completely sure if Rukario is a legendary or not. For me, the decision has nothing to do with the fact that he once belonged to a trainer. The fact that a legendary pokemon was used in a frontier battle dispels assumptions that legendaries can't be caught, or at least tamed. To me, Rukario doesn't look much like a legendary at all, and even though he has Hadou powers and is very agile and skilled, he just....doesn't have that...."legendary" feel that other legendaries possess. Plus, I'm not sure if he is the only Rukario in existance, although it seems to be that way. But hey, you never know, there could always be other Rukario around.
So, overall, I'm not completely sure if he is a legendary or not.