Melusine

She is typically described as a beautiful woman who is transformed, or reveals herself to be, a serpent or dragon from the waist down every Saturday

a woman above the waist and a serpent (or serpent-and-fish) below

She is typically described as a beautiful woman who is transformed, or reveals herself to be, a serpent or dragon from the waist down every Saturday

In French and broader medieval European folklore, Melusine is a water spirit who is a woman above the waist and a serpent (or serpent-and-fish) below, bound to a marriage vow that forbids her husband from seeing her true form on a fixed day.

Melusine is a legendary figure of French and medieval European folklore, most famously associated with the House of Lusignan in the Poitou region of western France.

In some tellings Raymondin's public denunciation of her as a serpent, rather than the spying itself, is what finally exiles her; upon the vow's violation she transforms fully into a winged serpent and flies from the castle, though she is said to return invisibly to nurse her youngest children at night and to reappear, shrieking, over Lusignan to foretell the death of a lord of the line or an impending change in its fortunes.

spies on her through a hole bored in the door and sees her serpent form while she bathes

In some tellings Raymondin's public denunciation of her as a serpent, rather than the spying itself, is what finally exiles her; upon the vow's violation she transforms fully into a winged serpent and flies from the castle, though she is said to return invisibly to nurse her youngest children at night and to reappear, shrieking, over Lusignan to foretell the death of a lord of the line or an impending change in its fortunes.

She is typically described as a beautiful woman who is transformed, or reveals herself to be, a serpent or dragon from the waist down every Saturday, and in some tellings she additionally bears wings and a fish-like tail associated with her legendary role as ancestress of the Lusignan line.

In the core legend she meets Raymondin of Poitou at a forest spring and agrees to marry him only on the condition that he never seek to see her on a Saturday, and in exchange for his oath she uses her supernatural powers to raise the fortunes of his house, most notably by having the castle of Lusignan itself built for him in a single night through magical means.

Melusine's principal power is her association with prophecy and supernatural building; beyond raising the castle of Lusignan overnight, she is credited in some tellings with founding or fortifying other strongholds attributed to her across the Poitou region.

In some tellings Raymondin's public denunciation of her as a serpent, rather than the spying itself, is what finally exiles her; upon the vow's violation she transforms fully into a winged serpent and flies from the castle, though she is said to return invisibly to nurse her youngest children at night and to reappear, shrieking, over Lusignan to foretell the death of a lord of the line or an impending change in its fortunes.