Why is the Republican Party represented by an elephant and the Democratic a donkey?

From the Smithsonian and Ann M

— Anonymous | Washington, D.C.

President Andrew Jackson, who was supported by the Democrats in 1828, earned the nickname “Jackass” for his stubbornness, says Jon Grinspan, curator of political history at the National Museum of American History. The image stuck to the Democrats and took off after the Civil War, when they were seen as the defeated party that wouldn’t accept its loss. Around the same time, cartoonist Thomas Nast started drawing a stumbling elephant to represent the Republican Party, once united by its abolitionist goal but struggling in the postwar years. Originally somewhat insulting, the two symbols were embraced in the early 20th century.