To the best of my knowledge, the term Ultraterrestrial was coined by author John A. Keel (The Mothman Prophecies) as a catch-all explanation for all paranormal phenomena. Computer Scientists and Ufologist Jacque Vallee also proposed a similar concept.
A quick google of the term will reveal that most people have no true understanding of the concept Keel was trying to communicate. For example, most references to Ultraterrestrials online are to Extraterrestrials, which is incorrect.
So what did Keel have in mind with regards to Ultraterrestrials? Let me do a quick breakdown:
-They may be composed of energy, inhabiting the spectrum (wavelength) of energy in which we cannot see.
-They me have evolved on this planet, though they are far older than the human race.
-They feed off of humans, though the implication is via the draining of energy and not the consumption of flesh. Keel uses the apt phrase, 'cattle of the gods' when referencing humans as a food source for Ultratterestrials.
-They are telepathic.
-They can temporarily take form or substance which they use to interact with humans.
-They are not extraterrestrials. Instead, Extraterrestrials are merely a 'mask' they wear.
-They are not Demons, as most religious people would interpret them as. The reason why is because 'Demon' is just another mask they wear, just as 'Angel' is.
-They are responsible for all paranormal phenomena. Everything from Bigfoot sightings, Ghosts, UFOs & Aliens, Marian Apparitions, Poltergeists, Fairies, Angels & Demons, Loch Ness Monster & sightings of mysterious creatures, among others, are just the machinations of Ultraterrestrials. The implication is that Ultraterrestrials will appear in such guises as a prelude to feeding off of humans. For example, an Ultraterrestrial may stage a Marian apparition to draw in the hordes of believers who subsequently lift their hands to heaven and out pour their hearts and energy to hungry Ultraterrestrials.
John Keel brought up some good questions that all readers should ponder with regard to paranormal phenomena. There's a lot of 'problems' with paranormal phenomena that the Ultraterrestrial Hypothesis addresses, though such hypothesis can never be proven one way or another.
Here's a list of things to ponder:
-Why no naked ghosts? (Do clothes have souls too?)
-Why does the Virgin Mary always appear beautiful and just like her statues? (The Real Mary lived in a time where there was practically no hygiene, no sunscreen, and let's face it, no dental care.)
-Why is a sulphur smell associated with demon encounters? This belief dates back to the time when people thought Hell was located deep underground, where sulphur is located. We know that Hell is not underground, yet people continue to report the smell of sulphur associated with demonic activity. Why?
-If Bigfoot is merely an unknown creature, then why are there reports of him having glowing eyes and of leaving behind a sulphur smell?
-Angelic encounters are a much beloved and cherished aspect of modern Christianity, yet how come no modern Christian ever realized that the 'beings' they are reporting do not meet the Biblical criteria for Angels?
-The same thing goes with Jesus. There are so many reports of visions or apparitions of Jesus, and they all tend to represent the Jesus we know through art. The problem is that nobody alive today, or even the artists themselves, has seen Jesus. Are Christians again being deceived?
-Why is it that Aliens are reported to be able to walk through walls like ghosts?
These are all fascinating questions which lend some credence to the Ultraterrestrial Hypothesis. But like all paranormal subjects, unless such an hypothesis can be tested and proved, it is ultimately pointless.