G1: The legendary is a enhanced, manmade clone of a mythical pokemon.
G2: The legendaries are two different, unrelated beings with their own myths.
G3: The legendaries hate each other and constantly seek dominance over one another.
G4: The legendaries are deities that keep other in check.
G5: The legendaries were once the same being, but split apart.
G6: The legendaries are related, but don't interact with each other.
G7: The legendaries are extradimensional beings.
G8: The legendaries are heroic siblings.