Scottish folklore divides the realm of Faerie and its fairy denizens into two categories: Seelie and Unseelie.
Seelie fairies, whose name stems from the same Scottish root word “silly,” meaning “happy,” are good-natured and generally associated with lightness, goodness, and benevolence to humans. It’s still wise to be cautious with a Seelie fairy, as some may play pranks and sometimes aren’t aware of the chaos they can cause. Unseelie fairies are more darkly inclined, appearing at night and wreaking havoc seemingly indiscriminately among mortals and their fellow fairies. They can sometimes grow fond of a human and treat him or her with kindness, but generally they are up to no good.
So be warned: The creature of moss and oak leaves that peers around a forest tree on your morning walk could be friend or foe, and the glitter-winged, dust-trailing sprite of your dreams might actually have razor-sharp teeth.