This is the world's smallest dog, standing at around 6-10" (15-23cm) tall. It is named after the northern Mexican state of Chihuahua. Along with chocolate, coffee, corn and tequila, the breed is one of the country's most successful exports. Most of the globe is familiar with the little guy.
The ancestor of the chihuahua was slightly bigger. This was the techichi, a favorite dog of the Aztec people. It apparently had a mystical significance too. The techichi were thought to be so loyal that they would guide their human companion's soul through the Underworld into safety. It would fight off all evil spirits in their path. As a result, the poor techichi was often sacrificed and burnt, along with their owner, so that the two could journey on together.
Even after the coming of Christianity to Mexico, this practise continued. Only now, the techichi were used as scapegoats. Cremating one in a human funeral pyre meant that the techihi dog would take on the human's sins. Thus its owner could carry on straight into Heaven.
The techichi, as a distinct breed, is extinct now, but its genes live on in the tiny chihuahua.
There is a theory that Spanish conquistadors brought with them a toy breed, like the Chinese crested dog. This mated with the techichi, in order to create a brand new hybrid, which eventually became the chihuahua. Others argue that images and other artifacts, including remains, have been found in Maya and Aztec archaeological sites. These obviously pre-date 1530, pointing to the fact that chihuahuas were in Mexico long before the Spanish came with their pets.
In 1904, the American Kennel Club officially registered the breed of toy dog, that the fashionable society ladies of Texas, Arizona and New Mexico were buying in Mexico. It was called the Chihuahua Dog, as in the breed from Chihuahua, though that quickly became shortened to a chihuahua. This is the moniker by which it is known throughout the world.
(For a moment there, it could have been called the Texas Dog or the Arizona Dog, which is where they were most prevalent in the USA, at the start of the 20th century. Today, they are stereotypically seen, in Mexico, as the pets of Manhattan's social elite, hence the dog's nickname here of 'New Yorker'.)
It is a popular misconception that chihuahuas have to be pampered by humans in order to survive. They were certainly the dogs of kings (the Aztec monarchs all owned one), but they have been known to form wild packs too. One such pack famously took up residence in the ruins of Tenochtitlán Palace, in Mexico City. Their ancestors had been abandoned after their Aztec owners, all members of the nobility, had been killed in the Spanish invasion of the 16th century.
Generations of this pack of chihuahuas lived there for three centuries, right up until the construction of the National Palace of Mexico, in 1850, near to the site. It's hard to build grand architecture, with dozens of 10" dogs yapping in defense of their patch, so, unfortunately, the wild chihuahuas were removed.
Chihuahuas are usually differentiated between two types: long-hair and smooth-hair. They are genetically exactly the same breed, but their fur is respectively long or short. (Smooth hair doesn't necessary mean 'smooth'. It can be velvetty or coarse, as long as it's short.) As show dogs, they shouldn't weigh more than 6lbs, though 2-4lbs is the ideal.
They are the favorite of the world's celebrities, with actresses and socialites in particular often seen with one under their arm. Famous chihuahua owners include: Marilyn Monroe, Sharon Osbourne, Paris Hilton, Hilary Duff and Cesar Millan. Chihuahuas often turn up in shows and in films, including 'Evil Con Carne', 'Invader Zim', 'Batman Returns', 'That's So Raven', 'The Soup', 'Legally Blonde' and 'Transformers'. They've also advertized Taco Bell. Let's not also forget that Ren Höek, the eponymous hero of 'Ren and Stimpy', is a chihuahua.
Boo Boo, the world's smallest, living, adult dog, is a chihuahua. She is 4" high, 6.5" long and weighs just 24oz. She can be found in Raceland, Kentucky, in the USA.