Not an animal, but an idiom. It means that a person's promise or oath is very reliable and trustworthy, as valuable as a thousand gold coins. This idiom comes from a plot in an ancient story in China. It tells the story of a politician and military commander named Cao Cao in the Three Kingdoms period. During the war, he sought advice from a think tank called Yu Xun. Yu Xun gave him good advice. Cao Cao immediately said that he would follow Yu Xun's suggestion and use precious gifts such as gold, silver and jewelry as sincerity and return. However, Yu Xun refused to accept it. He only asked Cao Cao to keep his promise and attack the enemy as soon as possible. At this time, Cao Cao solemnly said: I made a promise. Later, Cao Cao acted on Yu Xun's suggestion and won the victory. This story has been handed down and formed an idiom, emphasizing values such as credit, honesty and trustworthiness.