The best joke in the world, guess a zodiac.

It's a cow, because there is a story: at that time, the zodiac ranked first, saying that which animal arrived first, the mouse was small but cunning, and it was hidden in the cow. Just when the cow thought he was the first, the mouse ran down from him and became the first. In this way, rats come first, cows come second, and cows become jokes. That is, playing a joke on the best in the world.