The first female monster in the Toho’s kaiju gallery. Mothra is a divine giant moth that first appeared in 1961 in a serialized novel The Luminous Fairies and Mothra. She made her Godzilla film debut in the 1964 film, Mothra versus Godzilla.
Unlike Godzilla who switches between the hero and villian role, Mothra has most of the time fought to protect humanity. Except for probably one instance.
Mothra made debut in her own 1961 film, Mothra, in which she actually attacked human beings to protect her Shobijin fairies, a pair of tiny priestesses whom you will always see whenever Mothra makes an appearance. The Shobijin; literally translated as ‘Tiny Beauty’; serve as Mothra’s link to mankind. They are the only ones who have been able to summon and communicate with Mothra so far throughout the Godzilla film history. It remains to be seen if the upcoming Gareth Edwards film will see a Hollywood rendition of the Shobijin.
Through all the Godzilla films over the years, Mothra have at times played the opponent and an ally of Godzilla. Mothra is one of Toho’s most popular monsters and was aptly given the title of ‘Queen of Monsters’ by the film studio themselves. However, Mothra was not always female.
In the 1996 Rebirth of Mothra trilogy, the last of Mothra (Mothra Leo) was revealed to be a male, which may actually mean that the Mothra lineage have come to an end. But fortunately, the Mothra trilogy is not canon to the Godzilla universe. The trilogy, however, were one of the best kaiju films from Toho.
Mothra made her last Toho film appearance in Godzilla: Final Wars (2004) and was first hinted in the 2014 Legendary Pictures Godzilla film. She may have appeared in four films alongside Godzilla in King Ghidorah-related films, but the two monsters have only teamed up twice, in Ghidorah The Three-Headed Monster (1964) and Destroy All Monsters (1968).
Mothra fought on the side of humanity in Godzilla: Final Wars but did not outright fight alongside Godzilla. In Godzilla, Mothra and King Ghidorah: Giant Monsters All Out Attack, Mothra fought against Godzilla, which means their real last team up was in 1968.
References:
https://wikizilla.org/wiki/Mothra#Mothra
http://www.coolasscinema.com/2014/05/the-beauties-of-infant-island-look-back_7.html