I realize this could go a few different directions, but I know we all have some stories that left us freaked out or weirded out.
I have two, both deer hunting in Northern Idaho. Hiking down a skid road when I heard the most insane yipping and cayaying. It was heading right at me at speed, and I went from confused to fully assuming I was about to be whacked by coyotes/wolves/feral dogs. My hind brain took over and I dove behind a stump and threw my rifle across it, just in time to see a pair of Barred Owls come zooming through the brush, just making the most godawful racket you've ever heard. Took a few minutes to calm down after that.
Second was weirder and still unexplained. I packed a small muley about 4 miles back to the truck, arrived around 9pm, and found another guy waiting by my truck. Super nice, said he was just making sure I made it off the mountain cause it wasn't the safest area (his words). I asked him why, and he just put his finger to his lips and said "sit and listen". I was wrecked so I was happy to sit for a bit. After about 5 minutes, on the opposite side of the canyon I heard what I can only describe as a wounded elk squalling. Half bugle, high pitched, but changing tempo and pitch oddly. It went straight to the spine and made me want to bail. Right as that sound ended, the same type of call lit off on the mountain I had just came down. A third call answered the second from back to the west a good ways. These three calls went back and forth for all of 15-20 minutes before they just stopped with no warning. The other guy and I hadn't said a word the whole time we listened, fairly transfixed. Then he just smiled and me and said "this is why I waited. Not a good mountain to be on after dark". And off he drove. I have no idea what I heard that night (I'm not a big foot believer) but it was the most eerie, hair raising communication I've ever heard. I moved shortly after that (graduated from school and moved home) and I've never been back up there.