Bird trio: introduction to legendaries, proabily weather conditions. Based on real world legendary creatures
Mew: ancestor of all pokémon
Mewtwo: ultimate pokémon
Beast trio: mirrors the birds(?) a popular legend surrounds them though
Lugia: commander of storms, guardian of the seas, trio master
Ho-Oh: rebirth, guardian of the skies, trio master
Celebi: guarian of forests and time
Regi trio: rock-ice-iron age.
Eon duo. they go around
Weather trio: shaped the land and the seas, trio master keeps balance
Jirachi: shooting star, grants wishes
Deoxys: alien virus, supposed to excell in every field
Lake trio: emotion, willpower and knowledge
Creation trio: time, space, antimatter
Lunar duo: sides of the moon, good dreams/nightmares
Manaphy: guardian of the seas 2.0
Heatran: the core of the planet
Regigigas: pulled the continents, trio master
Shaymin: protector of flora
Arceus: creator of all, trio master
Victini: embodiment of victory
Swords of Justice: justice, mirror the three musketeers
Kami trio: winds, lightning, trio master. based on real world legends
Tao trio: truth, ideals and the lack of both
Meloetta: muse of the arts
Genesect: barely a legendary. Explaination given though.

EON DUO CONFIRMED FOR MOST USELESS AND POINTLESS LEGENDARIES EVER