A changeling, also historically referred to as an auf or oaf, is a human-like creature found throughout much of European folklore. A changeling was a substitute left by a supernatural being on kidnapping a human being. Sometimes the changeling was a 'stock', more often the changeling was a supernatural being made magically to look like the kidnapped human. Supernatural beings blamed for stealing children included fairies, demons, trolls, nereids and many others. Usually, the kidnapped human was a child; but there were cases, particularly in Scandinavia and Ireland, where adults were taken.
Der Wechselbalg by Henry Fuseli, 1781
The devil steals a baby, leaving a concealed changeling. Early 15th-century detail of "The Legend of St. Stephen" by Martino di Bartolomeo
Painting by John Bauer of two trolls with a human child they have raised
A fairy is a type of mythical being or legendary creature, generally described as anthropomorphic, found in the folklore of multiple European cultures, a form of spirit, often with metaphysical, supernatural, or preternatural qualities.
Fairy
1888 illustration by Luis Ricardo Falero of common modern depiction of a fairy with butterfly wings
Title page of a 1603 reprinting of Daemonologie
Illustration of a fairy by C. E. Brock