A Changeling

1. (mythology) 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. (informal, rare) An infant secretly exchanged with another infant by mistake or by human doing; swapling.
3. (science fiction and fantasy) An organism which can change shape to mimic others.
4. (obsolete) A simpleton; an idiot.
5. (obsolete) One apt to change; a waverer.

(fairy's child): auf (obsolete), oaf (obsolete)

(being that can change shape): shape-shifter

(a child exchanged for another): swapling