The Coming of Sin (1978) on Netflix

Lorna is a beautiful widow living all alone at her country chateau, with only her books and paintings for company. Then one day Triana, a wild gypsy girl who has been raised in an orphanage is brought her home by her friend Sally and her husband. She asks her to take care of her while they are away. She takes the girl under her wing, an sensuous relationship develops between them. Soon the pair are lovers. But Triana has a dark secret that manifests itself in frightening dreams of domination and humiliation. She has recurring nightmares of a naked man on horseback assaulting and abusing her. At the same time, the naked horseman begins to appear in reality. The girl, convinced that he represents her doom, resists him; but the artist is intrigued and a bizarre erotic triangle is established.