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Both folklore and archeological finds show that the breed originated in Mexico. The most common theory and most likely is that Chihuahuas are descended from the Techichi, a companion dog favored by the Toltec civilization in Mexico.

Historical records indicate that the Techichi hunted in packs. They can only be traced as far back as the ninth century but it is highly likely that this is the Chihuahua’s native Mexican ancestor^{[citation needed]}. Evidence of this is that the remains of dogs closely resembling, but slightly larger than the average Chihuahua have been found in such places as the Great Pyramid of Cholula, which dates back to the 2nd century BC and predates the 16th century. There is also evidence to suggest that the Techichi may also predate the Mayans^{[citation needed]}.

The Toltecs were conquered by the Aztecs, who believed that the Techichi held mystical powers.^[2] In terms of size, the present day Chihuahua is much smaller than its ancestors, a change thought to be due to the introduction of miniaturized Chinese dogs, such as the Chinese Crested Dog, into South America by the Spanish.

A progenitor of the breed was reputedly found in 1850 in old ruins near Casas Grandes in the Mexican state of Chihuahua from which the breed gets its name.^[3] The state borders with Texas, Arizona and New Mexico where Chihuahuas first rose to prominence and were further developed in the United States. Since that time, the Chihuahua has remained consistently popular as a breed, particularly in America when the breed was first recognized by the American Kennel Club in 1904. Genetic tests place the Chihuahua with other modern breeds originating in the 1800s.