The weather trio is, for lack of a better defining characteristic of the three, the name of the legendary trio made up of Groudon, Kyogre, and Rayquaza. They are referred to in the games as the super-ancient Pokémon (Japanese: 超古代ポケモンsuper-ancient Pokémon), although the Japanese anime has applied this term to Fossil Pokémon as well.
Although several non-legendary Pokémon, such as Tyranitar and Abomasnow, have abilities that can alter the weather conditions, and the Golduck family is capable of having an ability that can stop them altogether, Groudon, Kyogre, and Rayquaza's abilities were the first sight of these weather-changing ones in the games. This makes them the only legendary trio not to share one universal ability.
Unlike Pokémon with moves like Sunny Day and Rain Dance, where the weather changes to another for only five turns, the abilities of the legendary trio cause their weather effect for the remainder of the battle, or until the weather is changed again in some way. Rayquaza's, however, only lasts as long as it remains active, and removes the battle effects of any weather on the field, leaving it to just be a pretty-looking environmental effect.
They do not appear to have a master, though Rayquaza seems to serve as the leader of the three internally. It was shown quelling a violent confrontation between the other two Pokémon during the events of Pokémon Emerald.
Strangely, this is the only legendary trio whose members do not all have the same base stat total; Groudon and Kyogre share a total of 670, while Rayquaza's is 680. This fits with its role as a mediator between Groudon and Kyogre.
In the third version of Ruby and Sapphire, Kyogre and Groudon both appear in alternating caves after beating the Elite Four: Marine Cave (on Route 105, Route 125, Route 127, or Route 129) for Kyogre and Terra Cave (on Route 114, Route 115, Route 116, or Route 118) for Groudon. Rayquaza appears atop the Sky Pillar before the Elite Four, but only can be captured after the champion, Wallace, has been defeated.
All members of the weather trio can be obtained in the Hidden Tower after beating the Elite Four. Kyogre is obtainable in HeartGold, and Groudon is obtainable in SoulSilver. Once both Kyogre and Groudon are in one game, the player gets an opportunity to catch Rayquaza.
Kyogre, Groudon, and Rayquaza could be references to the Biblical Leviathan, a sea monster; Behemoth, a land beast; and Ziz, a giant bird, respectively.
Kyogre, Groudon, and Rayquaza could be references to the elemental spheres; Kyogre being the hydrosphere, Groudon being the lithosphere, and Rayquaza being the atmosphere.
No member of the weather trio has a signature move. They do, however, have "signature abilities".
In all other trios, all members are either single-typed or dual-typed Pokémon. The weather trio is the only trio whose members do not all fall into the same category (Groudon and Kyogre are single-type, Rayquaza is dual).
Rayquaza is the only member not to learn an OHKO move naturally.
Kyogre is the only member of this trio to be encountered after the main story line of Pokémon Mystery Dungeon.
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