The Dolphin (1987)

According to an Amazonian legend, every month, during the full moon, a Brazilian fishing village receives a mysterious guest: the Boto, who transforms into a human to seduce and be loved by women and hated by men. One of her conquests is the daughter of a fisherman, who has a son with the Boto. He constantly reappears to seduce her, and even when she marries, he continues to look for her. This provokes the ire of the husband, who wants to kill him anyway.