Changeling

n.	1.	One who, or that which, is left or taken in the place of another, as a child exchanged by fairies. Such, men do changelings call, so changed by fairies' theft. - Spenser. The changeling [a substituted writing] never known. - Shak.
	2.	A simpleton; an idiot. Changelings and fools of heaven, and thence shut out. Wildly we roam in discontent about.
	3.	One apt to change; a waverer.
a.	1.	Taken or left in place of another; changed.
	2.	Given to change; inconstant. Some are so studiously changeling. - Boyle.

Noun	1.	changeling - a person of subnormal intelligence Synonyms: cretin, half-wit, idiot, imbecile, moron, retard
	2.	changeling - a child secretly exchanged for another in infancy

agent, alternate, alternative, analogy, backup, change, comparison, copy, counterfeit, deputy, double, dummy, elf child, equal, equivalent, ersatz, exchange, fake, fill-in, ghost, ghostwriter, imitation, locum tenens, makeshift, metaphor, metonymy, next best thing, personnel, phony, pinch hitter, proxy, relief, replacement, representative, reserves, ringer, second string, secondary, sign, spares, stand-in, sub, substituent, substitute, substitution, succedaneum, superseder, supplanter, surrogate, symbol, synecdoche, third string, token, understudy, utility player, vicar, vice-president, vice-regent