ring-in

Noun

 * 1) A replacement made at the last minute, often in a sporting context.
 * 2) A racehorse or greyhound fraudulently substituted for another in a race.
 * 3) An outsider.
 * 4) A person pretending to be someone else; an imposter.
 * 1) A person pretending to be someone else; an imposter.