changeling (n.)

1550s, "one given to change," from change (n.) + diminutive suffix -ling. Meaning "person or thing left in place of one secretly taken" is from 1560s; specific reference to an infant or young child (usually stupid, strange, or ugly) superstitiously believed to have been left by the faeries in place of a beautiful or charming one they have stolen away is from 1580s. An earlier word for it was oaf or auf.