Octaman

Pier Angeli stars as Susan Lowry, a member of a scientific team that discovers a pool of unusual mutant octopus-like specimens in Mexico. The team soon encounters a six-foot humanoid creature, Octaman, who demands to have her "babies" returned and proves to be a formidable and murderous foe. After various attempts to capture or escape the creature, Susan Lowry ultimately kills it by shooting it at close range with a concealed handgun, saving herself and her colleagues from its clutches.