Changelings

A changeling is a faerie left in the place of a stolen human baby. Changelings are also deformed or otherwise ill-favored, but they conceal this with a power to transform their physical shape at will, usually into ferocious animals such as wolves.

And what becomes of the human child? It's usually raised by the faerie, though the child may be returned if the changeling can be induced to laugh or is treated so badly that its faerie parents are persuaded to take it back.