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.
An infant secretly exchanged with another infant deliberately or by mistake; a swapling.
A person or object (especially when regarded as inferior) secretly exchanged for something else.
An organism which can change shape to mimic others; a shape-shifter.
A werewolf
More generally, any sort of were-creature.