The Donkey and the Elephant

I always wondered how they became the symbols of the two parties. From Wikipedia...

In 1874, it was rumored that U. S. Grant would run for an unprecedented third term. As the rumors were surfacing, there was also a contemporary urban legend that several animals had escaped from the New York Zoo. Thomas Nast, the most popular and influential cartoonist of the time, took the opportunity to combine the two in a cartoon for The New Yorker magazine, representing the Republicans as elephants, docile but unmoveable when calm, unstoppable and destructive when excited. The cartoon, entitled "The Third Term Panic," depicted the Republican vote as an elephant running inexorably into a tar pit of inflation and chaos. Interestingly, the elephant was running away from the already established Democratic donkey, dressed in a lion's skin. This was Nast's take on the Democrats' view of Grant as Caesar, and their feeling that they had an obligation to play Brutus before he let the power of his office corrupt him.

The donkey predated Nast by three decades, when it was used during Andrew Jackson's campaign, initially by his opponents, calling him a 'jackass' for his populist policies. Well known as stubborn, however, Jackson decided to co-opt the mascot, and used it to his own advantage. After Jackson retired, he was still looked at as a party leader, even though the party refused to be led, and the 1837 cartoon "A Modern Baalim and his Ass" showed him leading a donkey which refused to follow. However, the donkey image was not popularized until the ubiquitous Nast adopted it, first depicting the party as a kicking donkey, attacking Lincoln's secretary of war Edwin Stanton even after his death in an 1870 cartoon for Harper's Weekly.

In other words, both animals were chosen for their negative qualities, such as stubbornness and willy-nilly destruction, and then adopted by the parties for their positive attributes, and neither party has been stubborn or destructive ever since.