Not really terrifying, but it kind of creeped me out after reading about it for a couple hours.
Apparently there's a lost photograph of a bunch of hunters/cowboys posing with the corpse of a so-called "Thunderbird", dating from the 1800s. It shows the body of either a massive (30-50 foot wingspan) bird or a pterosaur nailed to the side of a barn with wings outstretched, with several men standing in front of it indicating the scale. Despite many people remembering having seen the photo in print, nobody has yet been able to locate the original image.
The thing is, despite the fact that many people vividly remember seeing it, often in the same few publications, it doesn't exist. People will swear to god that they clearly remember the (very grainy/clear) photo with (3/5/6) (cowboys/farmers/hunters) standing in front of the (feathered bird/featherless pterodactyl) and can describe specific details. Some people name specific publications/page numbers, others just remember it from a vague book they read as a kid 30 years ago. It's a false memory, and a really infectious one. I own one of the books that it was supposedly in, and I'm now remembering stumbling across this story years ago, remembering the photo from the book, checking it and being surprised it wasn't there. But after reading about it so much, I'm no longer sure whether or not I just have a false memory of having a false memory.
Apart from that, I also ran across a lot of people sharing their own eyewitness accounts of anomolously giant birds, and it turns out it's kind of creepy even if you don't believe in it, especially since
they obviously do.