Groudon is the only Gen III Legendary to not appear in a movie.

Now, you may already be typing away in the comment section saying 'What about that one movie with the Flygon!?'

Well, before you post that comment, hear me out:

That Groudon was artificially made; a robot of sorts. It was not the real deal, not an actual Groudon.

But perhaps we may get a movie with its Primal.