In Godzilla, Mothra and King Ghidorah: Giant Monsters All-Out Attack, Mothra is portrayed as having been one of the three guardians of Yamato, originating 1,000 years before the events of the film.
Godzilla: Final Wars, which ignores the continuity of the previous film, establishes that Mothra protected the Earth 10,000 years ago from the cyborg Gigan.