Toei is Japan's most prolific producer of tokusatsu television. And have created more giant monsters and super robots than probably Toho and Tsuburaya Productions combined. However, their efforts are often overlooked, because many of their kaiju creations are of the expendable monsters-of-the-week variety. And from ridiculously 'toyetic' franchises, like Super Sentai and its American offshoot, Power Rangers.
And unlike Toho and Tsuburaya, Toei rarely revisits any of these characters, outside the marketable (and largely non-giant-sized) "Kamen Rider" villains.
And they're more traditional entries into the Japanese giant monster movie genre have been few and far between, with "The Magic Serpent" (1964) and "Legend of the Dinosaurs and Monster Birds" (1978) being the only two examples that some American fans can come up with...If any!
But during the late 1970's and very early 1980's, Toei did attempt three different, standard Kaiju films, though sadly non of them ever made it to filmed completion. Yes, we're entering the wonderfully obscure World of Unproduced / Unmade Giant Monster Movies!
Now very little information exist between these three following projects. And what is presented here comes from our own Keith Akein and Augusr Ragone. But for the sake of fandom awareness (and hopefully, additional information to be included in the replies), I'm creating a thread about it these three films right here.
"Kongorilla" (1977-1978)
A year or so before making the science fiction adventure "Message from Space" (1978) Toei first considered on making a fairly big budgeted kaiju film with a foreign source. Like Toho and Hammer's failed "Nessie" film (1976-1979), Toei and the British company Amicus (whose perhaps best known for their series of Edgar Rice Burroughs adaptations, staring Doug McClure) were going to make a giant ape film called "Kongorilla", to capitalize on the 1976 "King Kong" remake.
It wouldn't be too far-fetched to assume that Amicus, like Hammer with "Nessie", bailed out of the co-production and "Kongorilla" was eventually dropped, leading Toei to consider "Devil-Manta" next.
UPDATE: New information suggest that "Kongorilla", scripted by Tudor Gates, might have been in pre-production before the "King Kong" remake came into being. And unlike other giant primate films of the late 1970's, the 1976 film may have actually killed off "Kongorilla"
"Devil-Manta" (1978)
After "Kongorilla" was aborted, Toei went back on their own and wanted to do a Kaiju flick in the vein of a big-budget American disaster movie called "Devil-Manta". The film was to be about a 'giant flying manta ray from the void' stirring up some destructive mayhem. And would have been similar to the way 1978's "Message from Space" was done, production wise that is.
But the boxoffice success of "Star Wars" in the US and other territories in 1977, caused Toei's producers to shift gears, and went from monster-disaster epic, to a space opera take of the ancient Japanese novel "Saga of the Eight Dogs of Satomi" (also known as "Satomi Hakkenden").
Pictorial sketches featuring the unmade title monster attacking a metropolitan city and flying over smashed and smoking buildings can be seen in the 1987 book "Making of Toei Heroes".
"Mortal" (1979-1980)
And last but not least (probably); Toei was planning on making a film called "Mortal" back around 1979-1980. It was going to be co-produced with an obscure US company called Punch Productions. The story involved a 'simian-like creature' that is awakened by an eruption of Mt Fuji, and attacks Tokyo in a quest for food. The film was intended to be more horrifying than the typical Japanese monster movie.
UPDATE: New evidence suggest that "Mortal", like "Kongorilla", was in early development during the late 1970's, though also seemed to meet its end due to a stalling pre-production.