Mauro says:

While I cannot offer much
While I cannot offer much help in the field of ghostly research, I can give a few hints about both folklore and UFOlogy (my two main fields of interest).

UFO types since the ’40s: each period seems to have its own kind of UFOs/UAPs. Foo Fighters were mostly orbs of light taken to be secret weapons. At the end of the ’40s the type began to change into the classic, metallic flying saucers we are all familiar with. As the years went by UFOs/UAPs became more varied and increasingly larger, the cigar-shaped "mothership" seen by the Hills and many Latin American witnesses becoming an instant hit. The ’70s saw a resurgence of luminous UFOs in all shapes and colours. Finally the ’80s saw the arrival of the "Flying Triangles" and other crafts resembling high-tech vehicles made by human hands. It’s interesting to note that "metallic looking" UFOs/UAPs, capable of feats well beyond the known boundaries of physics as well as influencing the witnesses’ perception of reality, have been a constant all these years.

Alien types: if you accept Vallee’s theories the "Good Folk" of old are the aliens of today but let’s just stick to animated figures seen in conjuction with UFOs/UAPs. The Flying Machines seen at the close of the XIX century were crewed by absolutely normal-looking persons: they spoke English, French or Spanish. They were mostly dressed like train drivers or mechanics and talked in very down-to-earth technical terms, sometimes making absolutely normal requests (a bucket of water, a cold chisel etc). The Alien types seen after WWII are much different but there’s still a pattern to it. In ’50s America they were mostly the gentle, incredibly advanced, human-looking fellows popularized by Adamski (I know he was a fraud but bear with me), which later somehow transformed into the soulless "Greys" so popular nowadays. In South America Aliens tend to be aggressive and generally malevolent beings, be them hairy dwarves or muscular giants. In France they behave like gentle tourists (a few cases from the ’50s have them asking for directions to hapless witnesses!) or Carthesian philosophers. Also let’s not forget that sometimes Aliens take on them "messianic" traits: do as we tell or you’ll be destroyed, Mankind must change its ways or else… etc, somehow substituting religious figures.
 
Sorry for the obvious inaccuracies but I am writing from memory…