Toei president Shigeru Okada attended several film festivals and trade fairs in America and as he saw the way the film business was shifting toward blockbusters like Jaws, he felt that Japan should follow that trend. After all, who knew monster movies better than them?
Filming started the very same month that Jaws was released overseas. At the same time, everyone had grown obsessed with the Loch Ness Monster. Therefore, this was the perfect movie for Toei to appeal to not just the Japanese movie audience, but one across the world.
The film takes inspiration from the aforementioned shark movie, having attacks on boaters and swimmers and a build of the tension until the monster is unleashed. There’s even a gory scene where a headless horse is found hidden in a tree.
It turns out that not one but two kaiju are on the loose: a plesiosaurus and a rhamphorhynchus. They eventually battle and then fall into an erupting Mount Fuji.
Oddly enough, there are neither dinosaurs nor monster birds in a movie named Legend of Dinosaurs & Monster Birds. The plesiosaurus is a member of the sauropterygia and only a distant relation of dinosaurs while the rhamphorhynchus is a pterosaur, which is not a bird.
Perhaps even more strangely, this movie was a big deal in Russia and was, at one time, the 19th highest-grossing foreign film of all time in the USSR.
You can watch this on YouTube.