The origins of the donkey and elephant as symbols of the Democratic and Republican political parties in the U.S. were derived separately, but became synonymous with each other through the political cartoons of Thomas Nast, a caricaturist active in the latter half of the 19th century. Nast is also credited, along with Clement Clark Moore, with popularizing the modern image of Santa Claus in the United States. Thomas Nast first used the image of the donkey and elephant together to represent the Democratic and Republican parties, respectively, in 1870.

Source: Blue Star Strategies