Can a legendary Pokemon come back if you faint it?

i believe its possible for example i fainted dialga and then i went bak like 3days later and i seen it again.if this is false please tell me ok ;)

Gameguy100: That's probably because you didn't save the game after you fainted it.

blackhole23: actually with the release of Pokemon diamond and pearl some legendarys may faint and then may still be obtainable after a week or so though I don't which except for heatran....so I suggest you save before facing anyone of them.....

Miss_Macki: I fainted one of the lake Pokemon on Diamond over a year ago, It is still gone, What now?

Miss_Macki: Okay so they faint and are gone forever. Tip, have two of each version, so if you mess up like that you can fix it on the other game and trade it over (also you'd need another DS or DSI)