Melusine

There once was a King of Alba (now part of Scotland) named Elynas, who went hunting one day and came across a beautiful woman named Pressyne.  He fell in love with her and persuaded him to marry her.  She agreed, on condition that he never spy on her while she was bathing or birthing their children.  She gave birth to three beautiful daughters, the eldest named Melusine.  Everything went according to plan until Elynas couldn't contain his curiosity and spied on Pressyne as she bathed, finding out to his shock and horror that she was half-woman and half-beast, fish, etc.  She took their three daughters and disappeared, taking up residence on the Isle of Avalon.  Later the three girls determined to pay their father back for disrupting their family.  They caught Elynas and locked him in a mountain for all time.  Pressyne thought this very disrespectful and punished her daughters by condemning them to spend a day a week as a mermaid, undine or other supernatural creature.  Melusine happened to meet Raymond of Poitier (1115-1149, uncle of Eleanor of Acquitaine), Fulk of Anjou (970-1040-an early forbear of the Plantagenets), Guy of Lusignan's father, or an early Count/Duke of Luxembourg and began the cycle again, marrying him on condition he not spy on her in her bath.  Only this time, when he broke the condition she forgive him and stuck by him, until forced to hear Mass, or he insults her by being called a "serpent", "witch" or some such, and she turned into a serpent or worse and disappeared. 

Obviously, a quick check of the relevant records reveals that Fulk of Anjou's wives were Elizabeth of Vendome and later Hildegarde of Sundgau, both very mortal women.  Elizabeth's fate was bad enough, after being caught by her husband in an adulterous affair she was burned in the marketplace of Rouen in her wedding dress, but more on this later.  Guy of Lusignan's father Hugh (c 1110-1171) was married to a woman named Burgondie de Rancon and had a mother named Sarrasine.  Raymond of Poitier's wife was named Philippa and each of these women had an otherwise-normal life as a noble Medieval wife and mother.  No record that any of them looked like the lady on a Starbucks cup. 

So why the fairytale underpinnings of these royal houses?  It depends on which sources one checks.  Normal, in dealing with royal and noble men and women of that age, was a relative term.  If it's normal to fly into a rage and order the slaughter of 3,000 Saracens to get back at their Sultan for refusing demands, than Richard I was normal.  If its normal to leave one's wife and have a relationship with a woman who goes by the nickname of Dangereuse de Chatelherault and have rumors that one's niece (Eleanor) is one's mistress, than Raymond was normal.  Guy is a whole other blogpost and whether his life was mundane or normal I'll leave the reader to decide.  It was a rough and violent age, not a time for sissies.  These men were powerful, knew their way around swords and other weapons, and were in control of lands, castles and men.  To defy any of them was to risk one's life.  To have the power of life and death over those under their control was taken as a given.  But was with the other trappings of power, retinues, arms, jewels and wealth, it didn't hurt to have people believing one had a bit of the supernatural behind them, just a touch in the form of a distant ancestress.  Richard I said openly that his family was born of the Devil and would return to the Devil in due time.  Guy readily boasted that his mother had a undine (a water sprite similar to the Starbucks lady) in his mother's ancestry somewhere.  Fear is a potent part of power and, knowing that one's lord or king might be in league with darker elements added that fear.