Ringer
Etymology
From 19th-century American horse racing, where a "ringer" was a fast horse fraudulently substituted for a slower one of similar appearance to swindle bettors; the related "dead ringer" means an exact look-alike. The link to "ring" is debated, perhaps from the betting ring or from an older slang "ring" meaning to swap. It now names any ace slipped into an amateur contest.
noun · /RING-er/
A highly skilled competitor secretly brought in and passed off as an ordinary member of a team.
What it names
A specific fraud — disguising strength as averageness to beat opponents who think the match is fair.
Example
The office trivia team lost badly once their rivals brought in a ringer from the history department.
Related idea
A ringer hides real strength to win unfairly, whereas a dark horse hides nothing — its strength is simply unknown.
noun · /RING-er/
A person or thing that closely resembles another; an exact double (usually “a dead ringer”).
What it names
a near-identical look-alike
Example
He’s a dead ringer for his grandfather.
Related idea
Related to the imposter sense — both turn on one thing passing convincingly for another.
See also