Satan's Creature (1972)

One day, a shrimp fisherman catches a large tadpole. Dr. Yano discovers that the captured animal is not a tadpole, but a life form that originated in some chemical way in the polluted water of the bay. During experiments, the tadpole grows very rapidly and escapes, returning to the polluted water of the bay and growing into a monster. There happens to be another monster in the bay: Godzilla, in which the tadpole Hedra engages in a fight with.