Of course Europe is full of old tales of people cursed by evil enchanters to become various sorts of animals, some less cute than others.
But there are pockets where people aren’t getting turned into frogs or cats or wolves, but are getting turned into dragons or giant snakes. Since these creatures are considered so icky, it’s often incredibly beautiful girls or studly young knights who suffer this fate in legend. They are often confined by the spell to caves, or to twist around trees. Sometimes their curse makes them mindless and dangerous, but other times they retain human feelings.
Of course, the usual way to have a human turned into a dragon in European legend was greed plus a dragon-cursed treasure, as happened to Fafnir. These folks are definitely bad dragons. But Mandeville also has a story in Chapter 5 of the sunken Greek city of Cathailye, which fell to a dragon born of a necrophiliac union of the city’s prince with his dead wife. (“….there flew out an adder right hideous to see; the which as swithe flew about the city and the country, and soon after the city sank down.”)
There are also closely related cases where someone’s ancestor actually was some sort of were-snake fairy, most prominently the famous Melusine. She never did any harm, so we’ll put her on the side of good dragons.
“The Laily Worm and the Mackerel of the Sea” features a cursed young knight and lady, one of whom chooses to stay cursed and out of reach.
Mandeville’s closely related Greek island legend of a woman cursed to become a dragon (Travels, c. 4):
“And then pass men through the isles of Colcos and of Lango, of the which isles Ypocras was lord of. And some men say, that in the isle of Lango is yet the daughter of Ypocras, in form and likeness of a great dragon, that is a hundred fathom of length, as men say, for I have not seen her. And they of the isles call her Lady of the Land. And she lieth in an old castle, in a cave, and sheweth twice or thrice in the year, and she doth no harm to no man, but if men do her harm. And she was thus changed and transformed, from a fair damosel, into likeness of a dragon, by a goddess that was clept Diana. And men say, that she shall so endure in that form of a dragon, unto [the] time that a knight come, that is so hardy, that dare come to her and kiss her on the mouth; and then shall she turn again to her own kind, and be a woman again, but after that she shall not live long.
And it is not long sithen, that a knight of Rhodes, that was hardy and doughty in arms, said that he would kiss her. And when he was upon his courser, and went to the castle, and entered into the cave, the dragon lift up her head against him. And when the knight saw her in that form so hideous and so horrible he fled away. And the dragon bare the knight upon a rock, maugre his head; and from that rock, she cast him into the sea. And so was lost both horse and man.
And also a young man, that wist not of the dragon, went out of a ship, and went through the isle till that he came to the castle, and came into the cave, and went so long, till that he found a chamber; and there he saw a damosel that combed her head and looked in a mirror; and she had much treasure about her. And he trowed that she had been a common woman, that dwelled there to receive men to folly. And he abode, till the damosel saw the shadow of him in the mirror. And she turned her toward him, and asked him what he would? And he said, he would be her leman or paramour. And she asked him, if that he were a knight? And he said, nay. And then she said, that he might not be her leman; but she bade him go again unto his fellows, and make him knight, and come again upon the morrow, and she should come out of the cave before him, and then come and kiss her on the mouth and have no dread, – for I shall do thee no manner of harm, albeit that thou see me in likeness of a dragon; for though thou see me hideous and horrible to look on, I do thee to wit that it is made by enchantment; for without doubt, I am none other than thou seest now, a woman, and therefore dread thee nought. And if thou kiss me, thou shalt have all this treasure, and be my lord, and lord also of all the isle.
And he departed from her and went to his fellows to ship, and let make him knight and came again upon the morrow for to kiss this damosel. And when he saw her come out of the cave in form of a dragon, so hideous and so horrible, he had so great dread, that he fled again to the ship, and she followed him. And when she saw that he turned not again, she began to cry, as a thing that had much sorrow; and then she turned again into her cave. And anon the knight died. And sithen hitherward might no knight see her, but that he died anon. But when a knight cometh, that is so hardy to kiss her, he shall not die; but he shall turn the damosel into her right form and kindly shape, and he shall be lord of all the countries and isles abovesaid.
Btw, wyrm.org.uk is an excellent site for dragon folktales, particular UK ones.