According to Aztec belief, the Xoloitzcuintle dog, whose history dates 3,500 years, was created by the god of death to protect the living and guide the souls of the deceased through Mictlán, the underworld or the city of the dead.
The most important function that the Xoloitzcuintles were believed to fulfill was to help the souls cross a deep and mighty river that crosses the Mictlán.
Despite their goodness, they refused to help those who in life had treated animals badly, especially dogs, for which they would perish and would never be able to cross the underworld.
However, if the person had been generous to animals, including dogs, of course, the Xolo would gladly take their soul, lay it on his back, and carry it safely to the other side.
The term Xoloitzcuintle originates from Nahuatl: Xólotl, god of sunset and death, and itzcuintli, dog. Xólotl is the twin brother of Quetzalcóatl, who represents his opposite: darkness, underworld, death.
Because of their relationship with the god of death, people had to be kind, if they wanted to enjoy a grateful death without suffering.
The legend of the Xoloitzcuintle says that if the dog is black, it will not be able to take the souls to the other side of the river since its color indicates that it has submerged itself in the river too many times and has already guided enough souls to their destination. Similarly, if the dog is white or very light in color, it will not be able to cross the river either, as that means it is very young and has not yet been able to reach maturity to do so.
Only when they are a mottled gray color (which is usual for them), then they will be able to carry out this important and difficult task.
The Xoloitzcuintle was on the brink of extinction during European colonization, for two reasons: the conquerors found in it an unbeatable source of food for their expeditions; and also because it was sought to eliminate the pre-Hispanic religious traditions related to this animal.
Souce: Yucatan al Momento