The legendary Dragon- and Psychic-types Latios and Latias have appeared in a few games since their "Pokemon Sapphire" and "Ruby" debut. Both return in the "Pokemon ORAS" remakes with the added ability to Mega Evolve into Mega Latios and Mega Latias, granting both the ability to use Levitate for protection against Ground-types. Once you have these Pokemon, you can trade them into other games where they don't normally appear, except for Generation I and II games.
Tackling the Twins
As Dragon- and Psychic-types, the twins are tough opponents. Poison-, Ground-, Rock- and Steel-types are the only Pokemon who should enter battle against Latios or Latias; all other types either deal fatal damage or are too ineffective in battle. Be wary of your poison use, of course; you still can't capture either creature if it faints in battle from poison. When the bar is red, throw one of the stronger balls you have; regular Poke Balls probably won't contain a legendary, but you wouldn't want to waste a Master Ball either if you don't have to.
Pokemon Omega Ruby / Alpha Sapphire Locations
Latios appears in "Omega Ruby" while Latias appears in "Alpha Sapphire." You won't be able to find either one until Steven takes you to Southern Island after getting the Balance Badge in Petalburg City to teach you about multi-battles, which you'll have against either Courtney and Team Magma or Matt and Team Aqua. Steven only brings one Pokemon with him, but you won't face much trouble during this tutorial. When you win, the legendary dragon automatically joins your party -- a nice change of pace to the usual troubles of capturing one-time legendaries.
Pokemon Black 2 / White 2 Dreamyard
In "Pokemon Black 2" and "White 2," respectively, Latios and Latias appear in the Dreamyard, east of Striaton City. Whichever Pokemon you don't encounter in the Dreamyard won't be found anywhere else; for instance, you won't find Latios anywhere in "White 2." To get your Pokemon's twin, you'll need to trade for it or transfer it from an earlier game.
Pokemon HeartGold and SoulSilver Acquisition
Once it was possible to go to Pewter City and get either a Latios or Latias with an event item, the Enigma Stone. Without this now-unavailable key item, however, you're stuck waiting until Kanto where the legendaries roam. "SoulSilver" players will find Latios roaming while "HeartGold" players will see Latias roaming. As with "Black 2" and "White 2," having both dragons in your game requires trading.