Aggressive Chihuahuas are typically overprotective Chihuahuas. If your dog is happy and well-adjusted, it is probably not aggressive.
Animals that are kept in captivity tend to exhibit more behavioral problems than animals that are free to roam outside. One of the most common causes of aggression among captive animals is manifestations of territoriality. Protective tendencies coupled with any type of confinement can produce a desperate need for order and control, leading an animal to perceive everything around him or her as a potential threat to be eliminated at any cost…