Well, she isn't too elderly, she is 11 and in great physical health. Ally is a beagle mix adopted about nine years ago. She never was thrilled with the presence of other dogs, but got along just fine in the pack. But recently, she has been dangerously aggressive towards the family's other canines as well as my pet cat (who she ignored until now). It is both territorial, food, and displaced (like if she is barking at a bunny outside, and another dog comes to the window to check it out, she attacks the dog). Just today, when my cat came to me for some affection, Ally charged Tonks and chased her up the Christmas tree, and that has never happened before. She was growling, baring her teeth, and barking like crazy. Yesterday, she attacked our 15 year old lab as he walked to the water bowl, and he freaked out and defended himself (falling in the process, I could tell he was hurting after all the energy dissipated after the fight). I pulled them apart and got bitten by one of them in the process, but once Ally was in her crate, Noah was fine if not scared, his thick fur came in handy against her bites. The veterinarian rules out physical ailments. No broken teeth, arthritis, illness, hot spots, etc. that could cause her to act out because of pain. Also checked for a UTI, she is clean. Her eyesight and hearing is exceptional too. I honestly had to stop my father from taking her outside with a shotgun after she attacked his dog, when he came for a visit. But in the meantime all we can do is physically separate her in another room. We've tried to reintegrate Ally into the pack, but it only takes minutes before her fuse is lit. She has caused over 500$ in veterinary bills from fighting alone (not the health checkup to determine if she is hurting, but because of wounds she caused to the other dogs). I guess my problem is, is that I truly love that little dog. But at the same time, I can't handle the fact that this is going on. She is miserable if she is crated or kept in another room (even with another person), but if she is free then we have to hover to pull sparring dogs apart. Do you think it is wrong of me to consider asking the vet to euthanize her? Like I said, she is healthy, but around other animals is an unstable and unpredictable. Confining her for the rest of her life seems unfair, since it is her against three other dogs.