Huldra is a beautiful female creature from Norse folklore, she is said to have long blond hair and always wears a crown made of flowers. The only thing that separate her from humans is that she has the tail of a cow, which is why most men flee when they see her.
She was known to seduce young unmarried men and take them into the mountains, where she would not let them go unless they would marry her.

However, if a man did marry her (in a church in the name of god), tales tell that she would turn into an ugly woman. But in return she would gain the strength of ten men if not more, and she would lose her tail.








(on the right you can see another creature: Nøkken)Legends-

A boy in Tiveden went fishing, but he had no luck. Then he met a beautiful lady, and she was so dazzling that he felt he had to catch his breath. But, then he realized who she was, because he could see a fox's tail sticking out below the skirt. As he knew that it was forbidden to comment on the tail to the lady of the forest, if it were not done in the most polite manner, he bowed deeply and said in a soft voice, "Milady, I see that your petticoat shows below your skirt". The lady thanked him gracefully and hid her tail under her skirt; telling the boy to fish on the other side of the lake. That day, the boy had great luck with his fishing and he caught a fish every time he threw out the line; this was the huldra's recognition of his politeness.

In another story from Sigdal,when she avenged her pride on a young braggart she had sworn to marry, on the promise that he would not tell anybody of their secret relationship. The boy instead bragged about his bride for an entire year, and when they met again, she beat him around the ears with her cow’s tail. He lost his hearing and his wits for the rest of his life.

and here's a poem by Alan Hodgson named Huldra's wood http://www.odins-gift.com/poth/H/huldraswood.htm