According to Greek mythology, from the Night and Darkness were born many children, among which the god of sleep, Hypnos. Hypnos had three children called Morpheus, Phobetor and Phantasos. All of them, being Sleep’s sons, come to visit us when we are asleep. Sometimes they have fun while playing with the creation of our dreams.
Morpheus will usually get into our dreams in a human shape, while Phobetor (which means “terrible”) will take the form of animals and thus transform our dream into a nightmare. Phantasos (from which the word “fantasy” comes from) creates invented objects in our dreams, therefore the dreams he generates are unreal.
In Celtic mythology, Morgana is a special fairy. Neither good nor bad according to “human” morals, she is able to transform herself into whatever shape she wishes, and flying on raven’s wings she comes to our dreams at night and gives shape to them, not differently from Morpheus and his brothers.