The She Creature (1956)

When Andrea Talbott (Tom Conway), the assistant to carnival hypnotist Carlo Lombardi (Chester Morris), is herself hypnotized, the results are not positive. Lombardi somehow zaps Talbot's soul into the body of a prehistoric creature that lives below the waves. Lombardi commands the creature to the surface and uses her to destroy those who stand against him. Eventually, though, Talbot figures out a way to take back control of her will and fight her twisted employer.