Are You Checking Out My Tail?

A Huldra is a Scandinavian creature of myth. She lives in the forest and looks exactly like a beautiful human woman, just with one important distinction: She has a tail. This tail is in fact the most important telltale sign that you're not dealing with a normal human here, hence the Huldra is often depicted as not liking it when people see her tail.

Because the Huldra is usually depicted as a dangerously seductive creature; she lures humans to her with her enchanting song and then takes them away to her home underground, and they're never seen again. When doing this she's usually trying to pass herself off as human for as long as possible, and if her tail shows then, the victims might know something's up.

But the Huldra can be benevolent too. If a human treats her decently and is polite to her, she can often turn oit to be friendly and helpful.