Chihuahua

The smallest dog breed in the world, originating in the Mexican state of Chihuahua, possibly descended from the Techichi, a companion dog kept by pre-Columbian civilizations such as the Toltecs and later the Aztecs, who associated it with spiritual rituals.

Despite its tiny size, it has one of the most distinctive personalities among dog breeds: it's alert, brave enough to stand up to much larger dogs, and tends to form an intense bond with a single primary owner, often becoming jealous or wary of other people and animals.

It's a long-lived breed but physically fragile — some puppies' skull fontanelle doesn't fully close, requiring extra care to protect against head bumps. Because of this fragility, it's not the best choice for households with young children, and is better suited to adult or senior owners who can handle it gently.