changeling

Short definition from Wiktionary

Noun

1. 1
   In pre-modern European folklore: an infant of a magical creature that was secretly exchanged for a human infant. 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 demons, devils, or witches.
2. 2
   A person or object (especially when regarded as inferior) secretly exchanged for something else.
3. 3
   An infant secretly exchanged with another infant deliberately or by mistake; a swapling.
4. 4
   An organism which can change shape to mimic others; a shape-shifter.
5. 5
   A person apt to change their loyalty or thinking; a waverer.
6. 6
   An idiot, a simpleton.

Adjective

1. 1
   Changeable, fickle, inconstant, wavering.