Our Jack Russell is almost always friendly, but... Behaviour

Our 7 year old Jack Russell, Piccolo, is very nice to children, very sociable with people and is very docile and affectionate at home. The only baffling thing is that, once in a while, with male dogs, for no particular reason, he gets very aggressive. Just the other day, we visited friends who have a harmless little dog, a miniature rehpinscher, and he gets immediately very aggressive with it. Our friends said it was just because they were both on a leash and that we should free both in their backyard. I was not convinced but they talked me into it. For 5 minutes, all was peaceful, suddenly Piccolo attacked, and it was difficult to separate the two. The little dog got superficially wounded and need stitches from a dog veterinary! We have no idea why, since Piccolo is always peaceful and gentle!
When we walk in the park in town, he now and then gets aggressive with a large dog, which once tried to attack him when he was a puppy. He is also aggressive with dominant male Jack Russells. That we can understand.
Any ideas why he would get aggressive with a small, harmless male? What can be done? Squirting water in his face? Thanks for any suggestions.

I am sorry that I cannot help you but I not only have a Jack Russell but also I have never owned a male dog.

Hopefully someone will come along who can give you advice about Piccolo's behavior.

@6JRT's - this sounds like something you could help with.

Please do NOT squirt water in his face. He is seeing these dogs as a bad thing, and punishing him for it is going to make things even worse.
See's a dog, reacts, gets squirted in face. "The dogs must make the bad thing happen, I am right to shout at them!"

It's not uncommon for terriers to be dog intolerant, and that has absolutely nothing to do with how he acts around people. Muzzle train him so everyone can be safe while you work with him. He may not be able to interact with most dogs unmuzzled, but at least you can try and get him to ignore other dogs.
Distract him once he has seen the dog but before he reacts, then reward and praise all calm behaviour.

Yes, I agree with Blue Jay, and would also work on allowing him see the other dog, and then rewarding him for switching his attention onto myself. (Say, 'Watch me'. Dog obeys. You give a happy 'Yes' and treat him.). Use really tasty treats as a reward.
You could develop his focus at home by teaching him to Watch and Wait for a reward. Until he is more reliable, when you are on a walk, and see a dog that he doesn't like approaching, do not allow them to meet head to head. Just take you dog to one side, and make him calmly focus his attention on you while the other dog passes by. Always 'Yes' and reward him when he does the right thing.

Thanks for your feedback. I should explain, perhaps more precisely, than 95% of the time, he is friendly with other dogs. If another male dog gets aggressive, however, he responds accordingly, as he is absolutely fearless. He always gets along with females.

It is about 5% of the time that he responds aggressively to another dog. Strange was that with our friend's rather harmless dog, he got very aggressive.

We have tried to distract him when the large dog which once attacked him shows up, but, once he smells him, he does his Jekyll and Hyde routine...

Muzzling him would be an overkill, because he is always friendly, except for a couple of rare cases...

My previous dog hated most male pit bulls. Most of them hated him too. This lead to vicious fights that caused him or the other combatant severe injury. Unfortunately, he fought to kill . He lived to be almost 16 and he was in 3 severe fights, two on his own property where he was not in the wrong, but his hostility towards them caused fights that may not have been nearly as severe if he wasn't intent on killing them. In retrospect, I definitely would recommend trying to train this away instead of just saying it's rare and leaving it. My current female dog hates most other dogs and will viciously attack smaller breeds. A muzzle is needed with her since its MOST dogs she hates, but in public places, ANY dog that cannot be trusted 100% not to attack or has shown signs of viciousness needs to be muzzled. Carry mace to protect against any incoming dogs.

Is your dog still intact & is your friends little dog still intact as this could be one reason why your dog attacked the other dog [bitches will also attack dogs when they are in season if they are not ready to be mated] coming back to bitches being in season this can trigger normally calm male dogs to become alert & can/will attack other dogs when they pick up a bitches in season scent.

I have 6 JRT's 3 males & 3 females [mum dad & their 4 hooligans] they have all been neutered/sprayed & get along together, however one of my hooligans [Dottie] don't like other dogs & will attack for no reason what so ever, so she is always muzzled when on walkies [better safe then sorry] all of my 3 male dogs & my other 2 female dogs love all other dogs, but they will start to snarl/growl when a intact [dog/bitch] starts to eyeball them or tries to "hump" them.

ps your little dog as beautiful markings

Thanks! Piccolo with his favorite Teddy Bear...

Piccolo is gorgeous!

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Piccolo says thanks....

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