Latias is highly intelligent and capable of understanding human speech. It is covered with a glass-like down. The Pokémon enfolds its body with its down and refracts light to alter its appearance.
– Pokémon Alpha Sapphire Pokédex
Latios has the ability to make others see an image of what it has seen or imagines in its head. This Pokémon is intelligent and understands human speech.
– Pokémon Omega Ruby Pokédex
I’m putting Mega Latias and Mega Latios in the same post since they look basically the same (except the eyes). It’s kinda interesting how they both mega evolve to basically the same Pokemon.
Even though it’s in the game data for XY, Mega Latias/Latios aren’t actually available in those games. It isn’t until ORAS did they become available. In those games, you use them to Soar (even though they’re not even Flying types!). I really like the Soar mechanic, and wish it would make a come back.
Stat-wise, being legendaries, Latias and Latios already have good stats, so their mega forms enhance their OP stats, though neither gets any boost to their Speed; Latias is more focused on Defenses while Latios has more Attacking stats.