Thing is its impossible for me not to see both UFOs and Bigfoot's origins as being purely cultural, as there's hardly anything in either of them pre 1940's compared to their volume afterwards. In both cases they only start being seen regularly after its widely publicised that other people have seen them first, people report UFOs as saucers after the that's what the press told them to look out for, based in the first place on a corruption/misunderstanding of what Arnold said, yet still that's what people began to report seeing, it would be some coincidence that after the term flying disc/saucer is wrongly used saucer shape craft suddenly show up. Same with Bigfoot, after the Yeti hype of the 50's people suddenly start seeing more or less the same thing in the US. The damning point is that in both cases there is very little if any reports of either from earlier periods, of course there have been strange reports similar to both, but nothing to the volume that begin to appear after they've been popularised in the press.
The major difference to me though is that while the UFO phenomena definitely does begin in the 1940's, its also the time when aerospace development really explodes. This is likely to explain many of the sightings, a good example is the Avro Vulcan, which was flown over Northern England, even over urban areas prior to its existence being revealed to the public, prompting reports from those areas of large triangular UFOs, proving that eye witnesses while not being infallible are indeed capable of both identifying something strange, able to determine its shape and provide a reasonably accurate description of what they've seen.