Missing Episode: Astro-Monster was this for a while. Following Ghidorah and a delay involving distribution issues, Ebirah and Son went directly to TV in 1968, Destroy All Monsters managed to be released theatrically in 1969, and finally, the film was released as Monster Zero in 1970, four years after its intended U.S. release.
What Could Have Been: Mothra would have appeared along with Godzilla and Rodan, but the budget was getting out of hand so plans for that were scrapped.
There were plans to release the film in the U.S. as early as 1966, under the title Invasion of the Astro-Monsters, but they ultimately fell through. A work print of this earlier version was used as Toho's official "international" version of the film. The title card bares the original name, several SFX shots that got cut from Monster Zero are still intact and two lines were left undubbed.