The Republican elephant symbol was made popular by the political cartoonist Thomas Nast in the 1870s. Nast first used the elephant to represent the Republican Party in a cartoon published in Harper's Weekly in 1874, solidifying its association with the party.
The elephant initially appeared earlier, in 1864, in an illustration supporting Abraham Lincoln’s presidential campaign, where it symbolized strength and Union victories during the Civil War. However, it was Nast’s repeated use and famous cartoons, such as the 1874 “Third Term Panic,” that cemented the elephant as the Republican Party's mascot.
Hence, while the elephant was used earlier symbolically, Thomas Nast is credited as the creator and popularizer of the Republican elephant as a party symbol.
