Woman in White

Constance Welch, the Woman in White from 1.01 Pilot.

Woman in White A folk tale with many variations. The most famous is a Mexican story about La Llorona – The Weeping Woman. The story takes place long ago when a beautiful Indian princess, Dona Luisa de Loveros, fell in love with a handsome Mexican nobleman named Don Nuna de Montesclaros. The princess loved the nobleman deeply and had two children by him, but Montesclaros refused to marry her. When he finally deserted her and married another woman, Dona Luisa went mad with rage and stabbed her two children. Authorities found her wandering the street, sobbing, her clothes covered in blood. She was charged with infanticide and sent to the gallows. Ever since, it is said, the ghost of La Llorona walks the country at night in her bloody dress, crying out for her murdered children. If she finds any child, she's likely to carry it away with her to the nether regions, where her own spirit dwells. – Pad of Definitions (1.01 Pilot), Official Website

Apparently John encountered and took out a Woman in White outside Durant, Oklahoma, in 1991. John was of the belief that a Woman in White was the same kind of spirit that the Irish know as bean sidhe, or banshee.^BoM

Another story for a Woman in White is located near Dallas and known as the "Ghost of White Rock Lake." The story goes that a driver picks up a hitchhiker in a soaking wet white dress by the side of the road, who either disappears when they pass a cemetery, or who gives an address leading to an abandoned house.^BoM

For the pilot episode of Supernatural, the two versions of the Woman in White story were meshed together in the story of Constance Welch. Furthermore, the Constance Welch story draws on the theme of unfaithfulness (or sexual activity), which usually, in teen horror flicks, dooms the protagonist to die an untimely death.