[6] A study based on sequencing of ancient dog genomes, published in 2020, suggests that this pre-colonial ancestry survives in two Mexican breeds, to the extent of about 4% in the Chihuahua (and some 3% in the Xoloitzcuintli).

[7] Colonial records refer to small, nearly hairless dogs at the beginning of the nineteenth century; one claims that sixteenth-century conquistadores found them plentiful in the region later known as Chihuahua.

[3] Dogs of the older "deer" type, with a flat-topped head, more widely set eyes, larger ears, and longer, more slender legs, may still be registered, but the deer head is not considered a separate type in competition and a deer-head dog's digression from the breed standard is considered a fault.

[15] Pet Chihuahuas (those bred or purchased as companions rather than as show dogs) often range above these weights, even above 4.5 kg (10 lb), if they have large bone structures or are allowed to become overweight.

[1] In 2007 The Kennel Club of Great Britain amended its breed standard to disqualify merle dogs because of the health risks associated with the underlying gene.

[25]: 46 A 2018 study in Japan of pet cemetery data found the Chihuahua to have an average life expectancy of 11.8 years compared to 15.1 for crossbreeds and 13.7 overall.

[27][b] A 2024 UK study found a life expectancy of 11.8 years for the breed compared to an average of 12.7 for purebreeds and 12 for crossbreeds.