The scariest thing I have ever encountered in the woods (in the pitch dark before sun up) was..... Turkeys!
There is nothing quite like very quietly making your way through the woods toward your tree stand in the pitch dark, all by yourself, on a brisk quiet morning when, suddenly, the forest erupts with sound like trees are falling over and the forest is crumbling down on top of you. Turkeys flying/crashing through the branches of the trees right over head makes one hell of a racket. It took me a few minutes to understand what the sound had been. I think I pooped a little.
I can only imagine the noise that a flock of turkeys could make. Being surprised by flushing a single grouse will double your heart rate.
I heard of something really scary that glides along the edge of the woods. Said to be asexual/bisexual/genderfluid, it moves almost as fast as I can walk and makes an odd panting noise:
My turkey story isn't nearly as good.... early, but completely light out, and I only saw 1 turkey. Elk hunting. Windy. Lots of trees and ground cover. Somehow I got within a few yards when he exploded up and flew right by me into a tree ~30 yards away. Seemed 100x bigger flying by then when it landed in the tree.
Took me 10 minutes before the adrenaline abated enough to walk normally again. Soiled myself as well.
Rattlesnakes. Nothing like having a 4 foot diamondback start rattling 3 feet from your feet to get the old adrenaline going! Damned things are invisible in the dry grass. You don’t know they’re there until the rattle or strike.
Deer hunting in Vermont, climbing a steep hillside, trying not to make any noise in the near-dark dawn. A couple of partridge took off from a tree and the THUNDER of their wings, made me drop to my knees [to gather my wits] ...scared the holy beegeezus outta me.