It can be argued that any dog breed can be trained to be aggressive, and any dog breed can be trained to be passive. The fact is, particular dog breeds are often linked with particular traits. The pit bull is often viewed as an aggressive breed, and the golden retriever is often viewed as a passive, friendly breed. These views, however, are undue generalizations. Aggressive dogs come in many breeds and sizes. As it turns out, some of the most aggressive dog breeds are also the smallest breeds.
The dachshunds have a strong instinct to dig. Their name literally translates to "badger dog" in German. Dachshunds were bred to chase badgers out of holes, therefore, they are high-energy dogs. When dachshunds are unable to relieve their energy in a healthy manner, they can become irritable, obstinate and quick to bite.
The chihuahua is one of the smallest dog breeds, and therefore they become easily threatened by other dogs and humans. As with many small dog breeds, chihuahuas often display aggressive behavior for self-protection purposes, and may become suspicious of people other than their owners. Chihuahuas are not recommended for children because they can misinterpret a child's actions and defend themselves by biting.
Jack Russell Terrier
Jack Russell Terriers were bred for fox hunting and have retained strong hunting instincts. Jack Russell Terriers are high-energy dogs and like to chase, explore, bark and dig. If they do not receive enough exercise to occupy their energy, Jack Russell Terriers can become restless, destructive, and develop aggressive behavior.
Australian Cattle Dog
Australian cattle dogs were bred to protect cattle, and therefore retain a strong protective nature. They are avid guard dogs, and are extremely loyal and obedient to their owners. The Australian cattle dog's protective nature can lead to dominant and aggressive behavior, particularly with unfamiliar animals and persons.
Cocker spaniels were bred to hunt and track fowl. They are prone to roaming and hyperactivity. When cocker spaniel are not walked regularly, they can become restless and develop negative behaviors such as guarding of objects, barking, hyperactivity and aggressive behaviors.
Beagles are part of the hound group, and were bred to sniff out game. They have a predisposition to roaming, exploring, chewing and barking. Beagles are high-energy dogs and may become destructive when left alone. With lack of exercise and stimulation, beagles can develop negative behaviors such as excessive digging, chewing and biting.
Border collies were bred to herd livestock. Border collies are extremely intelligent dogs with a lot of energy to expend. When border collies are not challenged in both mind and body, they can become destructive. Border collies are not recommended to people who do not have ample time to spend with a dog. An unstimulated border collie can become irritable and develop aggressive behavior.
Chow chows were bred by the Chinese as working dogs, and home protectors. Chow chows can be aggressive around strangers, and have been know to be capable fighters.
The papillon was bred primarily for companionship. Papillons are extremely loyal to their owners. Often, they can be very possessive and suspicious of strangers. Their possessive nature can lead to aggressive and violent behavior.
English Springer Spaniel
Like its relative the cocker spaniel, the English springer spaniel was bred to hunt and track fowl. The English springer spaniel is intelligent, energetic and requires daily exercise. Generally, they are good with other pets but since they are natural hunters, English springer spaniels can develop aggressive behaviors when they are denied an outlet for their natural instincts.