Canon does not exactly define what makes a Legendary Pokémon in internal terms; however, it is important to note that the term “Legendary Pokémon” refers to two different things in the Japanese original: either Pokémon that are part of the records of myth (“mythical”) and are extremely rare, or Pokémon that are overall rare and have particular powers (“legendary”), distinction that lasts up until Generation Ⅴ.

For the purposes of the Suocéverse a Legendary Pokémon is any Pokémon that the franchise says it's Legendary _{(yeah, boring, I know)}.

Listing of specimens of Legendary Pokémon known in the Suocéverse:

Legendary Subset	Pokémon	Suocéverse identities
In franchise canon
Winged Mirages	Articuno - Zapdos - Moltres	Shamoutli Mirage Trio Emisre Mirage Trio Articuno (Sinnoh)
Other	Mewtwo, Mew
The cats dogs Gerbils	Raikou - Entei - Suicune	Johto Gerbil Trio Crown Gerbil Trio Bright of the Lake Emperor of the North
Tower Duo	Lugia – Ho-Oh	Lugia (Protector) Pleo Shadow Lugia Ho-Oh (Protector) Nix Shiranui
Giants / Golems	Regirock - Regice - Registeel Regieleki - Regidraco Regigigas	Hoenn Golem Trio Regice (Caledoria)
Eon Duo	Latias and Latios	Latii Species Ĵivadma The White Lady
Weather Trio / Super Ancient Pokémon	Kyogre - Groudon - Rayquaza	Warring Triad (Kyogre - Groudon - Rayquaza) Groudon (Folmo)
Others	Deoxys
Lake Trio	Mesprit - Uxie - Azelf	Lake Trio
Creation Trio	Dialga - Palkia - Giratina	Creation Trio Giratina the Second
Lunar Duo	Cresselia - Darkrai
Others	Heatran - Shaymin
Swords of Justice	Cobalion - Terrakion - Virizion Keldeo	Unova Muskedeer Trio Keldeo Cadrícea Muskedeer Trio
Therian Trio	Tornadus - Thundurus Landorus Enamorus
Tao Duo (Trio)	Reshiram - Zekrom Kyurem	Tao Duo (Reshiram, Zekrom)
Others	Meloetta - Genesect
Norse Trio	Xerneas - Yveltal Zygarde	Zygarde
Others	Diancie - Hoopa - Volcanion
Guardian Deities	Tapu Koko - Tapu Lele - Tapu Bulu - Tapu Fini
Cosmo Duo	Cosmog - Cosmoeg Solgaleo - Lunala	The Killer of the Sun The Aegess of Conquest
Others	Necrozma - Magearna - Marshadow - Zeraora
Meltan	Meltan - Melmetal
Fighters	Kubfu - Urshifu - Zarude
Legends of Galar	Zacian - Zamazenta
Steeds of the King	Glastrier - Spectrier Calyrex
Treasures of Ruin	Wo-Chien - Chien-Pao - Ting-Lu - Chi-Yu
Paradoxes	Koraidon - Miraidon	Trueno
Unloyal Three	Okidogi - Munkidori - Fezandipiti Ogerpon
Terapagos	Terapagos
Others	Celebi - Phione - Manaphy - Victini
Suocéverse-specific
Disbanded Heralds	»Uramaru (Pokémon) - »Rokotróa (Pokémon)
Powerforger	»Evevakarx (Pokémon)
Tao Duo		Original Dragon
Others	»Jakuavju (Pokémon)
The Big One		Arceus

Dev note: changes tracked in Legendary Pokémon (requires access).