Please tell.
If after all a changeling is left unaccounted for, they may grow up to become a person slow of wits and of poor manners, what was called an ouphe or oaf, while their human counterparts either pined away under the hill or lived quite happily among the dancing folk.
-Winter, the Celtic/Scottish lore chat
Basically a changeling who was used to replace a human baby, then grew up in the human household.