Does the franchise of "Bagi, the Monster of Mighty Nature" consist only of this one movie or is there anything more to it? Any sequels, series, mangas, visual novels, "spiritual sequels" (entirely different movie but taking strong inspiration in it) or any other paraphernalia related to Bagi?
From 1975-1976, Osamu Tezuka also wrote a manga titled Daichi no Kaoyaku Bagi (大地の顔役バギ), which would probably translate as "Bagi, the Boss of the Mighty Earth", with "boss" in the sense of "leader of a gang". See also the manga's article on Japanese Wikipedia (there are no writings on it in English that I can find).
It appears that this manga was incomplete at the time Tezuka stopped writing it. You can find the one volume Tezuka did complete on Amazon Japan. It does not look like anybody has translated this manga, ever.
I have not read this manga or watched the movie, but Japanese Wikipedia appears to suggest that the contents of the manga and movie are mostly unrelated - the Bagi in the manga is a jaguar, while the Bagi in the movie is a mountain lion-turned-human (I think?).