"It becomes problematic when prey aggression extends to other dogs, children and even adult humans."
©Scott Sheaffer, CPDT-KA, USA Dog Behavior
The most common behavior issue I see is aggression. Below I've listed three major categories of dog aggression with a brief description.
Fear Aggression. Simply stated, these dogs have learned and have been reinforced for using aggression as an offense-is-the-best-defense strategy for dealing with things that scare them. The prognosis for this kind of aggression can be quite good with proper guidance.
Control Related Aggression. This used to be called dominance aggression, but that name is misleading and it's not widely used anymore. These dogs are hard-wired to be in-charge when there is an owner leadership vacuum. Even with great leadership, these dogs can still be a handful. In many cases, with proper behavioral training, they can be rehabilitated and can live with their human family quite successfully. Treatment of this kind of aggression can be counter-intuitive; strong-armed punitive techniques usually make things worse.
Prey Aggression. Just about all dogs can't resist chasing the random rabbit or squirrel that runs in front of them - this is normal canine behavior. It becomes problematic when prey aggression extends to other dogs, children and even adult humans. The prognosis is frequently poor; fortunately, prey aggression directed at dogs or humans is rare.
If you have a dog with serious aggression issues, please seek the help of a qualified and knowledgeable dog trainer or behavior specialist.