A rather troubling incident occurred at our house today.
Mercy, a 4-year-old beagle mix, attacked Bear, a 1-year-old GSD/wolf hybrid. (He is about a quarter wolf; she's mostly beagle. Both are neutered) Bear is fine; Mercy is fine; but we are worried.
The circumstances were highly baffling. I only heard the attack; first a rapid-fire barking, loud and aggressive, from Mercy; then a scramble and scuffle. From my housemates, who saw it, I heard that Bear had been sitting chewing a rawhide, Mercy next to him; and she had attacked him seemingly without reason. Bear became frightened and peed all over the couch and floor. One housemate grabbed Bear; the other grabbed Mercy, "spanked" her, and dragged her to her owner's room, yelling all the while. (This particular housemate doesn't like Mercy.)
I came upstairs then, and took Mercy to her crate to cool down. She didn't seem to need it--she was quite calm when I came for her, and didn't attack Bear again when I led her past him on her leash. I put her in her crate, where she has been for the past half hour.
Bear and Mercy have a long-standing rivalry, to the point that we don't know which of them is top dog. Mercy used to hold that position quite firmly; but Bear is much bigger than her, very intelligent, and growing older; and now they get into frequent scuffles. Mercy is usually viewed as the instigator; she growls and bares her teeth at him sometimes, especially in the evenings when the dogs occupy the living room together.
(A note: I don't agree with hitting Mercy when she's been aggressive. She's been abused in the past, and she doesn't need any more of that.)
So, my questions are:
What reason could Mercy have had for attacking Bear? They say it was "out of the blue"--but Mercy isn't psychotic, and she must've had some reason. What could it have been?
This has to stop: Bear has very strong jaws, and could hurt Mercy; or else Mercy could scare him so much that his already fearful personality becomes too strong. (He is often afraid of dogs he doesn't know, and of new things.) What can we do to make sure this doesn't happen again?
Does dominance play a role here, or could it be something else?