The image below was shot at Mount Washington, New Hampshire in 1870 or 1871 (although some sources have dated it to 1880 or as late as 1896). As you can see, there's a picture of what appears to be cloud formation...with what appears to be a cigar shaped UFO flying in front of it!
Here is a zoomed in image of the UFO.
Nowhere on the list is a UFO mentioned though. You would think the photographers would have noted that if they saw a UFO...even if they didn't see it until after the photo was developed. It's also been said that the image is actually a bit of an illusion, not a distant image of a cloud but a close up image of a frost formation. So what is it really? Who knows for sure. There's a lot of recent UFO photographs that have debatable authenticity. This is just the earliest photo of debatable authenticity.