We are now on cockers 5,6 & 7 and our daughter has one. Sadly, number 4, a rescue boy we took in was "diagnosed" with Spaniel rage. All of the others and our springer have been fantastic dogs, so I think it is quite rare. Brodie, our number 4 had been in several homes before he came to us, including not making the grade as a prison dog.
He was an absolutely gorgeous boy, neutered and was black, tan and cream. When we first collected him him he was very calm and extremely well behaved, but over the 5 weeks we had him, he attacked various members of the family and also Minnie and Jess, our daughters dog. He would just snap for no reason and become very aggressive, his eyes glazed over, then almost like a light being switched on, he came out of it, seeming not to know what had happened. After the first week Pauline wanted me to take him back, but I loved him so much, I wanted to give him a chance. However, over the next 4 weeks it became more regular and for the first time in my life, I became frightened of him. On the Friday before I took him back, we were out on a walk with Minnie and Jess, when he suddenly attacked Jess and I couldn't get him off her. On the Saturday, we had to go to the vets as one of the three had been passing blood in their faeces and we didn't know which one. He had a go at the vet, who rolled her eyes, as if to say this dog is out of control. With a very heavy heart, on the Sunday I phoned the rescue and they agreed to have him back. I cried all the way back to Stoke, as I had bonded so well with him and I felt I had failed him, though I was assured I hadn't. He jumped out of the car when we reached there and was so good, as he was 95% of the time, but I couldn't take the risk of him attacking my 5 year old granddaughter. They had his assessed by a behaviourist and he attacked him, so in consultation with the vet, they decided he could not be re homed. I still have doubts that it was rage, I think it more likely his early upbringing, but whatever it was, he was PTS and when I found out I was devastated.
Sorry for the "War & Peace" but I just wanted to share my experience and say that if you get a cocker you can almost guarantee having a lovely family dog. My experience did not put me off, I got Dennis the same day as a puppy to join up with Minnie and rescued 5 year old Rosie one year ago. The only problem with cockers is they are so addictive, you can never have enough