Godzilla: Monster of Monsters

The mysterious Planet X appears when Pluto and Neptune switch positions in the solar system, and its inhabitants begin an attempt to conquer the Earth, using a legion of space monsters (though some of these creatures were in fact from Earth) as their primary attack force. The King of Monsters, Godzilla, joins forces with the guardian monster Mothra and the forces of Earth to repel the invasion forces. (wiki)