The image on the Mexican flag comes from an Aztec vision

In the fourteenth century, a group of Chichmecas (warrior nomads) called the Aztecs (or Mexicas) settled in Mexico when they saw an eagle (representing the sun) standing on a cactus (a symbol of the heart) clutching a snake (a symbol of the earth or Quetzalcoatl)—an image which is now depicted on the Mexican flag.