ghidorah (godzilla: the planet eater)

To that which brought the end of our civilization, we hesitate greatly to carelessly say its name. Words just have that much strength.
— Metphies in City on the Edge of Battle's epilogue.

Ghidorah was first alluded to in the 2018 film, Godzilla: City on the Edge of Battle, where Metphies says the name of an unspeakable destructive force to Haruo Sakaki, that being Ghidorah.

He will presumably play a role in the sequel to City on the Edge of Battle, Godzilla: The Planet Eater, and has been referenced in posters and teasers for the film.