u cn only catch latias in sapphire she will apper at random after the elite 4, latios is
in ruby only

After you beet the eleite 4 a tv show will say there has been a pokemon flyin over the
region. The pokemon will flee from battle unless you have a pokemon with scary face from
there catch it like a regular pokemon the best thing to do is watch your pokedex to see
where it moves from and stay in a area where it is most frequantly and eventually it will
apear but you must have the move scary face or it will run