Changeling

noun

1. In pre-modern European mythology, an infant that was secretly exchanged for a mother's own baby by an evil creature. (In British, Irish and Scandinavian mythology the exchanged infants were thought to be those of fairies, sprites or trolls; in other places, they were ascribed to witches, devils, or demons.)
2. An infant secretly exchanged with another infant by mistake or by human doing; swapling.
3. An organism which can change shape to mimic others.
4. A simpleton; an idiot.
5. One apt to change; a waverer.