All Legendaries that were obtainable are:
Articuno Zapdos Moltres Mewtwo Raikou(depending on Starter) Entei(depending on Starter) Suicune(depending on starter)
Legendaries only obtainable in events(not Obtainable anymore) Ho-Oh Lugia Deoxys
Here a locations of the obtainable Pokemon
http://serebii.net/red_green/legendary.shtml
The three legendary birds (Articuno, Moltres, Zapodos), Mewtwo, and one of the legendary beast depending on your starter.
