These are creatures that never were human. They may wear a human face, but their psychology is alien. Examples include the water-horses of Scottish folklore (kelpies who drown their lovers) or the fox-spirits ( kitsune ) of Japanese and Chinese literature.
The term "mortel" (French for fatal, deadly, or profound) in this context suggests relationships that are deeply intense, defining, and often marked by high stakes, including separation, death, or ultimate sacrifice. Unlike superficial depictions, these stories often emphasize: