We have 4 Jack Russel's. Francine is 10, We've had her all her life. Remy and Piper are brother and sister (from the same litter) and 16 months old. We've had them since they were 6 weeks. We got a dog from the pound, Mickey who is 2. We had Mickey for 6 months before we moved. No one has ever fought, little growls and snips here and there because the younger ones can be annoying. Remy and Piper are both a little large for their breed, so they often may run over the other dogs or play too hard. We moved into a much bigger house, bigger backyard that they run through, Nothing has changed besides the elevation which is now 4400+ and we were at 10 before. Remy has suddenly become aggressive to Mickey and when they fight they will fight until they bleed and it is extremely hard to break them up. Remy will start a fight if Mickey is eating (They free feed out of the same bowl). Running out the back door Remy will grab Mickey and start attacking him. Remy is a good 10 pounds more and much stronger so Mickey can't hold himself up that great. Remy has never tried to attack him before or acted like this. Its getting to the point where vet trips are almost a must. Ever since we got Remy he has always acted like he has ADHD. He is very antsy, He jumps, gets in the way, He will bite your hands as you're walking, climbs all over you, Pretty much has no boundaries and whatever we do will only stick for one minute then he resets. We have tried putting him in tight shirts so he might feel a little more comfort, But that only works for so little time. We never lived in a community where we could walk him so he's not very well leash trained to go on walks, but its a working progress. We are considering a shock collar for whenever he does something bad. He's never acted like this, So I would really love to know what could have triggered all this aggression. All answers are appreciated! Thanks in advance.