Perhaps the most misunderstood Pokémon are the so-called, and with good reason, "legendary" Pokémon. It is a matter of much debate as to whether or not there are multiple specimens of each species or a single, immortal specimen. While lesser Pokémon have large populations that can sustain themselves, legendary Pokémon do not. While several species of legendary Pokémon are known to have more than one member, there is only one legendary Pokémon that was known to be truly unique: Mewtwo. However, this uniqueness did not last forever. With Mewtwo's approval, several other Mewtwo were created. However, these Mewtwo were not the same as the original: they were less powerful and less intelligent. While they were capable of feelings and emotions, like all Pokémon are, Mewtwo did not wish for his children, as he called them, to have the ability to hate like he once did, as it would spell disaster for the world.
It has been proposed, by Mewtwo himself, no less, that both cases are correct. He believes that each legendary specie of Pokémon has a single, immortal member that gave birth to less powerful and less intelligent members of its own species to give Pokémon trainers a chance to capture and befriend these legendary Pokémon. Since capturing Giratina, for example, would leave the Distorted World defenseless, and capturing Dialga and Palkia would potentially destroy space and time, it would make sense for less powerful members to be created.
Articuno
Zapdos
Moltres
Mewtwo
Mew
Raikou
Entei
Suicune
Lugia
Ho-Oh
Celebi
Regirock
Regice
Registeel
Latias
Latios
Kyogre
Groudon
Rayquaza
Jirachi
Deoxys
Uxie
Mesprit
Azelf
Dialga
Palkia
Heatran
Regigigas
Giratina
Cresselia
Manaphy
Darkrai
Shaymin
Arceus
Victini
Cobalion
Terrakion
Virizion
Tornadus
Thundurus
Reshiram
Zekrom
Landorus
Kyurem
Keldeo
Meloetta
Genesect
Xerneas
Yveltal
Zygarde
Diancie
Volcanion
Hoopa