If your Chihuahua is completely healthy, the most likely reason that he is acting this way is because of a dog’s natural instinct to be the “alpha dog”. Our cute, little domesticated dogs still interpret things in the canine way.
The house is the den, all humans and all animals are the pack and the leader – the one in charge – is the Alpha. If it is not 100% clear who holds this position, a Chihuahua may take it upon himself to claim the spot. When this happens, he may act very aggressively.
In multi-dog households, there also needs to be an “alpha dog”. Among the pack of dogs, one will be in charge. If done correctly he will be an example and role model to the other pets and answer only to his human leader.
All of the dogs can be best friends; however one will indeed be the leader. If this hierarchy among the pets in not clear, a Chihuahua that tries to claim the spot may behave in an aggressive manner.
In these cases, time does not fix things if the environment remains the same. Without intervention, tension mounts. As these stresses build, a Chihuahua will continue to test his humans to see who will back down and who – if anyone – will step in to correct him.
An interesting facet of this is that for most dogs that are convinced that they are indeed the Alpha will be normally be very calm. Only when someone comes close to their food or their resting place or does not do as the dog wishes, will they then growl or bite to show that they are “king”.