Johnny-come-lately
Johnny-come-lately {n.}
Someone new in a place or group; newcomer; also: a new person who takes an active part in group affairs before the group has accepted him; upstart.
Everybody was amazed when a Johnny-come-lately beat the old favorite in the race.
When it looked as though Mr. Brown had a good chance of winning, many Johnny-come-latelies began to support him.
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Source: A Dictionary of American Idioms