Changeling

In European folklore and folk belief, a changeling is the offspring of a fairy, troll, elf or other legendary creature that has been secretly left in the place of a human child. The apparent changeling could also be a stock, a glamorized piece of wood that would soon appear to grow sick and die. The motivation for this conduct stems from the desire to have a human servant, the love of a human child, or malice. Most often it was thought that faeries exchanged the children, and simple charms, such as an inverted coat, were thought to ward them off.

For further information, please see the Wikipedia entry: Changeling