Changeling

According to folklore, a fairy would secretly exchange a mortal infant with that of the fairy kind. The fairy baby was called a changeling. Sometimes the stolen babies were returned to the families, especially when a person could expose the true nature of the changeling. The changeling existed in all Celtic folklore and fairy tales.

In Scottish Gaelic tradition, the changeling was called tàcharan or umaidh. In Irish folklore, it was corpán sidhe, síodhbradh or síofra; and in Manx it was Ihianoo shee. The Welsh called them plentyn a neidiwyd am arall.

Usually the fairy babies were sickly. One way to recognise them was to place them on a fire and chant a formula. If it was a changeling it would leave, climbing up the chimney.

According to Yeats, the stolen baby would live in a place of full "good living and music and mirth".

So far, in Celtic mythology I have come across mortal babies being stolen by fairies, but not replaced with fairy infants.