Changelings

Changelings are creatures spoken of in many folklore, fantasy and fairy tales from across the world - they are regarded as creatures that are placed into mortal homes by fairies or demons, who proceed to kidnap the original child of the home.

Legendary Traits
Changelings are often said to exhibit odd behavior or have traits that are not normally found in human young (such as having extra toes): some tales also spoke of Changelings that had voracious appetites, unnatural knowledge and imp-like traits that made them more mischievious and/or disturbing to their mortal "parents".

To ward off Changelings people would often put small trinkets in beside their infants and later it was considered wise to baptize one's young as it was believed creatures such as trolls would only take unbaptized children.

Reality
The tales of Changelings were also a contributor to very real neglect, cruelty and even murder amongst certain individuals - as many children (often disabled and/or of ill health) suspected of being "changelings" were subjected to harsh treatment by superstitious communities wishing to rid themselves of what they believed to be a malevolent or unwanted intruder.