Notes on Legendary PokemonIt should first be noted that only a few of the legendaries are truly unique(meaning only one of them will or has ever existed).More Like This
Many legendaries of a particular kind may exist in the same time frame. I.E. A new Entei is said to be born from a newly formed volcano.
Alternatively, though many legendaries of a particular kind may not exist in the same time frame, the legendaries who are not unique are also not immortal and may die or be reborn. I.E. The celebi of the past may not be the same celebi of the present, even in they are time travellers.
The only truly unique legendaries are: Arceus, Dialga, Palkia, Giratina, Regigigas, Jirachi
These pokémon are also unique because they were man-made: Mewtwo, Genesect
 Mirage Trio
Moltres -- Fire -- Created land with Ice-comboZapdos -- Electricity -- Energy created currentsArticuno -- Ice -- Created land with Fire-combo
 Beast Trio