Hm... I thought I posted here already... oh well!
The category of Legendary Pokemon is not such an easy thing to define. Other than them not being owned by a trainer (though we have seen one of them used in battle during the Battle Frontier), there's not much to define them.
They are supposed to be extremely powerful, but so can a Charizard if you train it enough. I don't think that power should be a criteria.
I think of legendary pokemon as pokemon that:
1. Revolve around a legend or legends. Uh, that should be a no-brainer.
2. Are hardly ever glimpsed by human eyes. Unless in extreme situations, or by trainers that are very VERY good.
3. Have some sort of connection to the planet. Mew, Ho-oh, Deoxys, etc, all have some connection to the way the planet works. Whether it be the origins of said planet, an element of the planet, the weather, etc. And I don't mean like the meek connection that all pokemon have, I mean something important, like a Snorunt can't just randomly start a huge blizzard that encompasses a mountain, and the planet won't start dying without it. It has to be a big connection.
So, um, Rukario is a pokemon that is owned by Aaron, but it's never said if he's rare or not. For all we know he could be quite common around the area, but is the only one that's been trained up a bit. Also, Rukario's role seems to be very similar to what Ash's roles usually are in the movies. He's there to save the Tree of Origin/Beginning and pretty much has the same role as Aaron and Ash. To me, that's not a legendary pokemon; that's a legendary hero. So, in conclusion, according to me, Rukario is not a legendary pokemon.
Though he is very cool and I did enjoy the movie. ^^;