ring-in
noun1
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.
- "But I couldn't get into an underground contract party because I'm a ring-tail, or ring-in." That means an outsider, one not born in Broken Hill.1967, Elspeth Huxley, Their Shining Eldorado: A Journey through Australia, page 97:
4
A person pretending to be someone else; an imposter.