Thomas Nast is perhaps 19th Century best US political cartoonist. He came up with the idea of using animals to represent political parties. In his cartoons, the Democratic Party was a donkey and the Republican Party an elephant. Nast also helped to develop the character, Uncle Sam, to represent the United States. 
The Off Year, Harper's Weekly, (1877) Wood engraving
The American River Ganges. Harper’s Weekly (1871). Wood engraving.
(here is a short video about Thomas Nast)