As Jaimie has mentioned, it sounds like your dog is becoming fear aggressive. I am dealing with this problem myself, and have consulted with many experts. There are a few important things that you must understand. First is that you shouldn't be harsh or use choke chains etc. to try to correct this problem. Try not to yell or get overly upset when something sets him off. Dogs have two responses to something that they are afraid of: Fight(Bite) or Flight (Run away) He would probably prefer to run from something that he is afraid of, but if he is trapped such as being on a leash, than you are leaving him with only one option: Fight or Bite. You must assume at this time that he would bite someone so that you are extra careful during this retraining phase. It will take a combination of training and management at this point to help him. Please try to remember also to keep a cool head, because it will get on your nerves and upset you most certainly. A dog will read your stress and frustration and it will make him more stressed. Try to stay calm. OK-let's get down to business. Start to hand-feed your dog each meal for the next year while practicing obedience-this will establish your alpha role. Also, try to bring your dog for hard exercise (ie. fetch, or swimming) in a secluded or enclosed area (ie. empty ball field) where there aren't other dogs or people. When people come to your house, you may want to crate him, or put him in a room until the people are in and sitting down (until things are calm) then let him approach people on his own terms. You can also give visitors treats. You may want to incorporate a muzzle to be used during short periods of known stress just for protection. As far as introducing him to new situations, you must always give him space. With dogs, space is everything. Something that bothers him at 20 feet, may not bother him at 30 feet. Dogs have their own comfort distance for everything. Start just in your driveway (at the back) and always keep treats on hand- the goal is to have the dog react less and less to things over time as you are able to move closer and closer. Only work for about 15 minutes a day or your dog will become tired and irritable. For example, start in the back of your driveway, a good distance from the road, and when something passes that you know will stress him, try to get him to sit and if the distance is good than he won' t react and you can give a treat. If he reacts than you know you have to move further back. Once you know his comfort distance, then as he is calm you can move a bit closer step by step each day-take it slow. If he does react to something try to laugh and use your happy voice. If you yell or punish, he will just associate that "bike or person or dog" with him getting in trouble and it will be even harder to retrain. Always make light of it and move away from the stressor. Call him to you and get him doing some basic commands to get him more focused on you. A fear aggressive dog is not the same as a dominance aggressive dog, however, they do have some commonalities-such as the way the appear. A fear aggressive dog may appear to want to attack everything, but usually, they are afraid and are doing the only thing they know how to do. (On-leash fear aggression is very common) The last thing for now is that you probably should make a vet appointment to see if there are other underlying reasons for his behaviour. The vet may chose to use medication to help, but I was advised by vets specialized in aggression issues not to use anxiety and prozac type medications as it can remove the fear, and then you just have an aggressive dog without the fear inhibition. You can however try some bach flower remedies-these are natural flower essences that help people and pets with emotional problems. You probably should stop taking him for walks down the street for a while until the problem is more manageable. You also may want to refrain from any obedience class-it will not help your problem and can make things worse. Many trainers do not know how to deal with fear aggressive dogs. A class environment is very stressful. If you can find a knowledgeable trainer giving a small class (2-4 dogs) with much space then give it a try, but be careful he may not be ready for that yet. (Trust me I speak from experience) Work with him in a low stress environment, and if you must take him for walks down the street, try to move to the opposite side of the road (make an arc) from people and other dogs and bikes etc. (Don't let anyone approach-remember that a reaching hand is a threat to some dogs-and they will bite) You have to be alert and see the trigger before he does. Your dog is not a terrible dog who deserves to euthanized, he just needs some extra care and work on your part. I think you should get some books on fear aggression and reactivity in dogs, and consult a dog behaviorist specialized in aggression if you can.
This is a complicated problem to deal with, but can be done, with some understanding and lots of patience on your part. You have to understand that he may never be a "normal" dog, but can still live a happy life. It's worth a try. If you have any questions please let me know.
Good Luck.
