Any animal.
The actual scariest stuff pretty much boils down to aggressive dogs being aggressive dogs.
For example: I saw a woman lose control of her pit-mix. The dog rushed over to one of our neighbors, unprovoked, and immediately latched on to her leg and started thrashing it. The dog’s owner was freaking out screeching which seemed to agitate the dog rather than dissuade it from attacking. There was so much blood, but I think the only long term physical damage was scars. However, I only ever saw the victim once or twice after that, she stopped going out on walks after that event.
If you’re willing to stretch the definition of scariest and seen:
Deer are super annoying sometimes. One of their behaviors is that they make a super loud snort-whistle noise when alarmed. So if you’re outside and unaware of their presence, and especially if you’re not expecting a super loud and almost alien-like snort-whistle coming from the woods right next to you, there’s a good chance it’s going to startle the crap out of you.
Elk in road. No lights, no moon, no other cars headlights. Like a big furry wall just showed up in my headlights. I chose a direction to swerve and now I’m not allowed to test my luck ever again.
A friend was on house arrest, staying in a guest bedroom at his uncle’s house. He had a small black cat with white paws named Psyche, but the rule was Psyche had to stay in his room.
One day Psyche got out and ran down two flights of stairs to the basement. I went down for him to get her and bring her back up. I coaxed her toward me, then I scruffed her and was holding her scruff (also supporting under her) tightly as I walked back upstairs.
I thought to myself “I’m being too hard on this cat” and I relaxed the grip I had on her scruff just slightly. She saw this as weakness and immediately went into battle mode. The moment I relaxed my grip slightly, she took one swipe at my other hand with her paw and then bolted when I dropped her.
The thing that terrified me was this: her paw went through my hand like an scoop through ice cream. All the claws worked as one to just scoop a little chunk of flesh out of the pad of my hand. Like, it was a 3D wound.
It was the first time I understood what a cat’s paw is. Those individual claws aren’t the weapon. The paw is the weapon. And when a cat’s paw is swiping, my flesh is like whipped cream in its path.
A moose just stood there being a moose. Moose are terrifying.
Raven opened my cage and ate my pet parakeet