Latias is classified as the Eon Pokémon (along with her brother Latios), and is listed as #380 in the National Pokédex. It can Mega Evolve in Gen VI if holding a Latiasite. Some basic stats:
Height: 4'7" (1.4m) [Mega Latias: 5'11" (1.8m)]
Weight: 88.2 lbs (40.0kg) [Mega Latias: 114.6 lbs (52.0kg)]
Habitat: Water's edge
Gender ratio: Always female
Breeding Group: Undiscovered [can't breed]
Growth rate: Slow (1,250,000 EXP to level 100)
As a Dragon/Psychic-type, Latias takes half damage from Electric, Fighting, Fire, Grass, Psychic, and Water-type attacks. It takes double damage from Bug, Dark, Dragon, Fairy, Ghost, and Ice-type attacks. Due to her ability, Levitate, Latias is immune to Ground-type attacks, though that can be negated through various means.
Latias's name supposedly comes from the Latin word "latere," meaning "to lie hidden." I suppose it's a good enough explanation for Latias and Latios's names. Latias's name is the same in pretty much every localization; the original Japanese is ラティアス (Ratiasu).
Latias is a tricky Pokémon to find: it's not entirely a version exclusive Pokémon, it's more that you need a special key item that you're only going to get from events if you're not playing the right game. (More about that over here.) Here are the games it can be found in:
- Generation III: Available in Ruby (requires Eon Ticket), Sapphire, Emerald; trade to FireRed, LeafGreen, Colosseum, and XD
- Generation IV: Available in HeartGold and SoulSilver (requires Enigma Stone); trade to Diamond, Pearl, and Platinum
- Generation V: Available in White 2; trade to Black, White, and Black 2
- Generation VI: Available in Omega Ruby (requires Eon Ticket) and Alpha Sapphire; trade to X and Y
Latias has also shown up in a bunch of the side games throughout the years:
- Pinball: Ruby & Sapphire
- Mystery Dungeon Red/Blue Rescue Team
- Mystery Dungeon Explorers of Time/Darkness
- Ranger: Guardian Signs
- Rumble Blast
- PokéPark Wii: Pikachu's Adventure
Given the high number of Pokémon spinoff games, it's entirely possible I missed a few — if you know of any other games Latias appears in, please drop me a line!
Latias (along with Latios) also appears in Super Smash Bros. Brawl and Super Smash Bros for 3DS/Wii U as a Pokéball Pokémon, which I think means she randomly shows up to help now and then. (Don't look at me, I've never played Smash in my life and I only vaguely understand how it works. Nothing against it, just never had a chance to play.)