I have an 8 yr. old bitch (who is actually an aunt to Jett) who has the same behavior. However with her, it has nothing to do with bitch on bitch, rather she attacks dogs that are submissive/passive. Her current favorite victim of my dogs is my 5 yr. old Toss (male), who sounds very similar to Fly. He never even defends himself, let alone fights back, and I'm quite sure he does nothing to incite her - other than to exist. Basically, she will attack dogs that she thinks she can get away with it with. When I got my now 3 yr. old Taff at 5 mos., she attacked him. He gave back as good as he got, and she's left him alone since. I've found no solution other than management. She is always crated in the house, and I supervise closely outside. For her, the primary triggers are dooways, gateways, and smallish areas. I always keep myself between her and Toss, and warn her off when I see her giving him a hard look. Once she attacks, she has to be physically removed and carried away (fortunately she only weighs about 30 lbs.!), as nothing else will stop her. BTW, she started with it at 7 wks., when she attacked my mom's old (male) JRT. At 4 mos. she attacked the old (male) GP that lived at the farm where I was working. She also hates my 14 yr. old bitch Dot, although only attacks when Dot is under the weather. She has no issues at all with my 6 yr. old bitch.