Before you read this, watch this video:
Okay so you may notice I put a comment there with the EXACT information I put here. You can read that, instead of coming back here if you want, because it's the same thing. Don't go there and see that comment and accuse me of being some sort of stealer, please. I wrote that comment. Also, there are a couple of theorists in the comment section, so be sure to read all of them!
I do not own the video or artwork or music. The video belongs to Aliyah Mcdermott. The music and artwork are by various artists. Find her here: https://www.youtube.com/channel/UCRKvXWsKdBA8FN_i3xUwnQQ
Onward to the theory
Before I watched this, I assumed(and still am) that it's Breeze.
~Arrow leaves the pack and has pups with Bella
~Storm leaves the pack because she's tired of being accused
~Dart leaves the pack with a few other dogs
~Breeze is usually near the scenes of the attacks, and in one book it talks about someone putting glass in Storm's bedding, and Breeze says loudly, "It's not like Storm herself could have done this." Storm then thinks , "I wish she would stop saying that."
~every time something bad is about to happen, Storm sleep walks, but usually the horrible events take place when she's awake
~Martha is killed by Blade
~Wiggle is killed by Blade
~Whisper is killed by our Mystery Dog
~ Blade had plenty of time to resurface while everyone was around, but never did, and she haunts Storm's dreams, so we KNOW she's dead
~Fang is killed by Blade
~(spoiler warning for Alpha's Tale) Alpha's (AKA Dog) neck is broke from impact one ice, so he's dead
~Breeze supported blaming Moon for stealing and hoarding prey
~when Moon fell into a ditch, a dog made it to her before Storm could, recognized her without looking at her, and kicked rocks onto Moon
~after Storm reached Moon, whom was stuck in a ditch, Breeze reached them second
~Breeze was always quick to assume ~when the rabbit incident happened, Beetle and Someone else, I forgot who, were napping, and Breeze was with the pups. They were the only dogs in camp, giving Breeze plenty of time to steal a rabbit from the pack, and smear it's blood as quickly as possible, then run to the pond, throw it in, come back while the pups were asleep, and make an insane bark
~Breeze suggested leaving the pack to Storm, then they have a play fight and Storm "hurts" Breeze
~don't forget the dead fierce dog we saw
My final conclusion: Breeze has teamed up with the remaining fierce dogs from Blade's pack, acting as a spy for them. The fierce dogs help her in some of her schemes, like the night Breeze came back covered in scratches. The fierce dogs must have done that to her and told her to lie. If she is working with them, her main priority would be to get rid of Storm. Think about it, Storm is the only one in the pack who could face the fierce dog pack. When Storm goes to Breeze, asking for support, Breeze suggests Storm leaving the pack for awhile.Then, in order to support her opinion of Storm leaving the pack for awhile, she fakes hurting a paw. This leaves a perfect chance for Fierce dogs to attack camp, or any other dog who left the camp, because a divided pack is a weaker pack. Breeze most likely felt sorry for Storm when she left, however, like how Lucky felt sorry for Mulch after he made Mulch take the blame on something. It's also most likely great to have a spy in the pack who could stay near the pups, a last resort of all else fails.
Okay I'm done spoiling.
If you have anymore proof that it might have been Breeze, please tell me!