Melusine
Elinas, King of Albania was out hunting in the forest and came across a beautiful woman singing beside a spring. She was Pressyne, a fay (faery). The King asked her to marry him and she agreed but first he had to promise never to visit her chambers while she birthed or bathed.
She gave birth to triplets; Melusine, Melior, and Palatyne. On hearing the news the king burst into her chambers as she was bathing her daughters. After breaking his promise, Pressyne took her 3 daughters and and went to the lost isle of Avalon.
On their 15th birthday, Melusine asked what their father had done wrong. After hearing his crime she was set on revenge and along with her sisters, captured the King and sealed him in a mountain along with his riches.
Upon hearing what her daughters had done, Pressyne condemned her daughter Melusine to take the form of a serpent (in other stories a fish) from the waist down every Saturday. If she met a husband who would agree never to see her on a Saturday then she would remain in human form. Melusine married a man named Raymond who agreed to her conditions.
However, one Saturday Raymond became curious and peeked through the keyhole of Melusine’s chambers while she was bathing. He saw her in her half human form and was filled with rage, bursting in and calling her a serpent. Melusine then transformed into a dragon and flew out the window never to be seen again.