One of the most interesting categories of spirit being found in folklore throughout the world is the Trickster. Known by a range of names (Puck, Coyote, Raven, Anansi,, and Loki to name a few) these beings, usually gods but not always, are an odd example of a type of being that can be very hard on humans but is not generally seen as wicked.
The shape differs between cultures the core concept itself is consistent. Tricksters are usually seen as teachers who seek out individuals and help them learn a spiritual lesson usually by making them look foolish. Their targets tend to be people who are notable for their great pride and arrogance with the deception being aimed at teaching them some humility although the tricks can take other forms as well.
A good example of the sort of trickster the traditional Trickster figure plays can be found in a Native American legend about the god Coyote. It’s said that at one time the frog people had a large dam that held back all of the water. Supposedly Coyote came to them to trade a dentalia shell (actually something he made from the bones of a dear he had slain) for a drink of water. The story goes that Coyote pretended to drink for a long time all the while digging at the dam from the inside while the frog people were unaware. Finally after they had grown suspicious he rose up just in time for the dam to break freeing the water so that all could share in it. He did it not out of malice or cruelty but because all people deserved to have water not just the frog people.
While most tricksters are seen as benevolent (if mischievous) there is a notable exception, the Norse god Loki. In Norse mythology Loki is seen as a very dark figure guilty of several misdeeds, including arranging the murder of the god Baldur although Loki set up another to commit the crime instead of doing it himself, who will when the time comes help to usher in Ragnarok the Norse end of the world. He is an interesting example though in his own right. While Loki commits some evil deeds and is slated to bring about the end he features prominently as a hero in several other tales using his cleverness to protect Asgard and even help retrieve his brother Thor’s hammer when it is taken. Even his role in Ragnarok raises some questions about whether it is something he chooses purely out of malice or the product of the cruelty he and his children suffered at the hands of the other gods.
Coyote Legends: http://www.native-languages.org/legends-coyote.htm
Loki Legends: http://library.thinkquest.org/25326/Viking/loki.htm