Chihuahuas are extremely protective of their family or owners. They will bark at noises if they feel it's suspicious, where some other breeds wouldn't think anything of it.
Also interesting: "...there is sufficient evidence to prove that it is without doubt an ancient breed originating from Pre-Columbian Mexico, and it is believed to outdate any other breed of dog in the Americas. "
http://en.wikipedia.org/wiki/Chihuahua_(dog)