https://en.wikipedia.org/wiki/Dark_horse
Dark horse concept was first applied to James K. Polk, a relatively unknown Tennessee politician who won the Democratic Party's 1844 presidential nomination over a host of better-known candidates. Polk won the nomination on the ninth ballot at his party's national nominating convention, and went on to become the country's eleventh president.
Other successful dark horse candidates for the United States presidency include:
Franklin Pierce, chosen as the Democratic nominee and later elected the fourteenth president in 1852.
Abraham Lincoln, chosen as the Republican nominee and elected as the sixteenth president in 1860.
Rutherford B. Hayes, elected the nineteenth president in 1876.
James A. Garfield, elected the twentieth president in 1880.
Warren G. Harding, Senator from Ohio, elected the twenty-ninth president in 1920 after his surprise nomination.
Harry S. Truman, Vice-President and former Senator from Missouri and thirty-third president, was virtually unknown to the American people when he succeeded President Franklin D. Roosevelt in 1945. Truman was considered a lame duck President with no chance of winning against Republican nominee and New York Governor Thomas E. Dewey, yet managed to shock the world by emerging victorious in the 1948 United States presidential election - widely considered one of the biggest upsets in American history.
Jimmy Carter, former Governor of Georgia elected the thirty-ninth president in 1976; in the beginning of that same year, Carter was relatively unknown outside his home state of Georgia but went on to win the nomination over rivals with more national prominence. At the 1976 Democratic National Convention Carter made a joke of his obscurity beginning his speech saying "My name is Jimmy Carter, and I'm running for President."
Barack Obama, the Junior Senator from Illinois, who had captivated the Democratic Convention on behalf of John Kerry in 2004, was still relatively unknown to the American people when he entered into the 2008 Democratic Party presidential primaries but emerged from obscurity to narrowly edge out the heavily favored Hillary Clinton for the nomination. Obama would be elected president, becoming the nation's first African-American president.
Donald Trump, a real estate investor and reality television personality, defeated 15 established rivals for the Republican nomination before defeating former Secretary of State Hillary Clinton in the general election despite losing the popular vote by over 2.8 million votes. Trump had never held political office prior to his presidency, but had been running or espousing to run for president since 1999