I feel for you. I have 3 GSD. Two I got from attica animal shelter last year. The only major fight they have had I broke up, and it cost me a trip to the hospital for a bite for myself and a trip to the vet for 2 of the three dogs.That's what we thought also. But since all of this has started, the Mrs. has done a lot of research, joined several Beagle specific groups, and has talked to many, many people, to find out that this is fairly common in Beagles. Some start out fine and then after sometimes years, they hate each other. People have come home to bloody messes from Beagles fighting while they're away.
Personally, I've never met an aggressive Beagle, until Gunner. During the research and discussions with other Beagle owners, we've had reassurance of what we suspected all along; that Gunner was abused, and he was never properly socialized.
I feel really bad for him, and if we ever find him a good home, I will surely miss him.