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 examples
The Mirage Birds	Articuno - Zapdos - Moltres	Shamoutli Mirage Trio - Emisre Mirage Trio - Articuno (Sinnoh)
G1 individuals	Mewtwo - Mew
The cats dogs Gerbils	Raikou - Entei - Suicune	Johto Gerbil Trio - Crown Gerbil Trio - Suicune (Caledoria) Emperor of the North
Lugia	Lugia	Lugia (Protector) - Pleo - Shadow Lugia
Ho-Oh	Ho-Oh	Ho-Oh (Protector) - Nix - Shiranui
Celebi
The Golem Trio	Regirock - Regice - Registeel Regigigas	Hoenn Golem Trio - Regice (Caledoria)
The Latii	Latias / Latios	Latii Species
The Weather Trio	Kyogre - Groudon - Rayquaza	Warring Triad (Kyogre - Groudon - Rayquaza)
Jirachi
Deoxys	Deoxys
The Lake Trio	Mesprit - Uxie - Azelf	Lake Trio
The Creation Trio	Dialga - Palkia - Giratina	Creation Trio - Giratina the Second
The Night Duo	Cresselia - Darkrai
G4 individuals	Heatran Phione - Manaphy Shaymin
Arceus
The Muskedeer	Cobalion - Terrakion - Virizion Keldeo	Unova Muskedeer Trio - Keldeo - Cadrícea Muskedeer Trio
The Therian Trio	Tornadus - Thundurus Landorus
The Tao Duo	Reshiram - Zekrom Kyurem	Tao Duo (Reshiram, Zekrom) - Kyurem
G5 individuals	Victini Meloetta Genesect
The Norse Trio	Xerneas - Yveltal Zygarde	Zygarde
G6 individuals	Diancie Hoopa Volcanion
The Cosmo Duo	Cosmog - Cosmoeg Solgaleo - Lunala
Tapus
G7 individuals	Necrozma Magearna Marshadow Zeraora
Other	Meltan
Suocéverse-specific
Unown		Unown
The Tao Duo		Original Dragon
The Original Life		Miju - Drao - Feyt

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