The Godzilla knockoff was originally a prehistoric monster that wreaked havoc on Japan but Gamera’s origin was eventually retconned to make him a good guy similar to Mothra. Description from l7world.com. I searched for this on bing.com/images
MST3K #302 - GAMERA - The movie opens as a mysterious jet is flying over the Arctic, so a general orders fighters to shoot it down. Unfortunately, the jet was carrying a nuclear bomb and the atomic blast from its crash awakens a prehistoric monster, the giant flying turtle named Gamera.