Escudo Nacional de Mexico

The coat of arms of Mexico is a national symbol of Mexico and depicts a Mexican eagle perched on a prickly pear cactus devouring a rattlesnake. The design is rooted in the legend that the Aztec people would know where to build their city once they saw an eagle eating a snake on top of a lake. The image has been an important symbol of Mexican politics and culture for centuries. To the people of Tenochtitlan, this symbol had strong religious connotations, and to the Europeans, it came to symbolize the triumph of good over evil.

The Mexican law on the National Arms, Flag, and Anthem regulates the name, the design and use of the arms. There they are officially called "coat of arms", even if there is no heraldic shield and therefore, according to the rules of heraldry, it is not a traditional "coat of arms" and more precisely a "national emblem" instead.

Source: Wikipedia