Wikipedia
In Norse mythology, a fylgja is a spirit who accompanies a person in connection to their fate or fortune. The word, fylgja (or plural fylgjur) means “to accompany” similar to that of the Irish Fetch, it can also mean “afterbirth of a child” meaning that the afterbirth and the fylgja are connected. In some instances, the fylgja can take on the form of the animal that shows itself when a baby is born or as the creature that eats the afterbirth. In some literature and sagas, the fylgjur can take the form of mice, dogs, foxes, cats, birds of prey, or carrion eaters because these were animals that would typically eat such afterbirths. Other ideas of fylgjur are that the animals reflect the character of the person they represent. Men who were viewed as a leader would often have fylgja to show their true character. This means that if they had a “tame nature”, their fylgja would typically be an ox, goat, or boar. If they had an “untame nature” they would have fylgjur such as; a fox, wolf, deer, bear, eagle, falcon, leopard, lion, or a serpent. In “Dreams in Icelandic Tradition" by Turville-Petre, it discussed commonalities between the various animals such as an evil wizard or sorcerer’s fylgja would be a fox because they are sly and hiding something, or an enemy is depicted as a wolf. Particularly in The Story of Howard the Halt otherwise known as Hárvarðar saga Ísfirðings, the character Atli has a dream about eighteen wolves running towards him with a vixen as their leader, predicting that he would be attacked by an army with a sorcerer at the front.
Fylgjur could also “mark transformations between human and animal” or shape shifting. In Egil’s Saga, there was lots of references to both the characters Egil and Skallagrim transforming into wolves or bears, as well as examples of shape shifting in the Saga of King Hrolf Kraki, where Bodvar Bjarki turns into a bear during a battle as a last stand. These transformations are often hinted to when sagas discuss berserkers who transform into animals or imitate animalistic characteristics.
Fylgjur usually appear in the form of an animal or a human and commonly appear during sleep, but the sagas relate that they could appear while a person is awake as well, and that seeing one's fylgja is an omen of one's impending death. However, when fylgjur appear in the form of women, they are then supposedly guardian spirits for people or clans ( ættir). According to Else Mundal, the women fylgja could also be considered a dís, a ghost or goddess that is attached to fate. Both Andy Orchard and Rudolf Simek note parallels between the concept of the hamingja—a personification of a family's or individual's fortune—and the fylgja. An example of such an occurrence would be in Gisli Surrson's Saga where the main character, Gisli,is visited by two beautiful women, one who is trying to bring good fortune and one that is trying to edge him towards violence. These two women could represent the women ancestors of Gisli's family ties, such as the ties between his wife Aud and his sister Thordis, relating to the idea of the hamingja and dís.