The term ultraterrestrial covers creatures, beings and entities that are not of this dimension, examples that by some considered as ultraterrestrials are angels, demons, orbs, rods, ghosts and even some mythological plants and beasts like unicorns, fay, centaurs and leprechauns that may at some point have been able to cross into our dimension, or been seen by some from our realm as they blended through parallel dimensional dreams. Some even think that alien UFO visitors might be of a ultraterrestrial origin rather than extraterrestrials.
Angels and Demons as Ultraterrestrials
These opposite forces of good and evil seem to have several traits that could be explained as visitations of a multi-dimensional type. Both seem to be able to enter our realm in more of a metaphysical state as they seem to be beings of force and energy.
Mythological Beasts and Beings
In this category you have all the large beings mentioned within different cultures mythology (unicorns, dragons, mermaids, cyclops, minotaurs, pegasus, leprechauns, e'epa, menehune, leviathan, satyrs, and some semi-inteligent animals, like the cat like Ullgolthr and Rallurgh.) These are according to some, beings that actually used to co-existed in our reality but was later prevented from freely crossing the dimensional veil. (For other explanations of Mythological beasts read the entry Unicorns, Camelopardalis and other Cryptids.)
Plants, Vermin and Other Lesser Creatures
Much like in the category above these co-exist in the borderland of the dimensions and sometimes or used to be present in our realm before the veil was closed. Examples of dimensional beings of this order are the elusive rods and orbs, invisible to the human eye but sometimes captured on film. Most of these are easily debunked as bugs or specs of dust, though some might be dimensional visitors.
Ghosts and Hauntings
Ghosts are similar to angels and demons, all of them being incorporeal, though ghosts do not possess the power of higher beings, they do have the ability to seemingly exist in our realm, at least partially. This would also explain why ghosts are so hard to capture since they are not fully part of our world anymore, and many of them have not crossed over the veil.
Dreams and Astral projections
In some cultures dreams are thought to literally be traveling into a separate world, another dimension made out of pure thoughts, this would make the dreamer an ultraterrestrial traveler. Astral projections are by definition ultraterrestial travel as ones soul enters into the Astral dimensional plane that are theorized to blend in with our own reality, making the astral traveler present in our world but still unattached in spirit.
The Veil is the very fabric of time, energy and space thought to separate our realm from parallel multidimensional realms that intersects our own reality, psychics are thought to be sensitive to the weaknesses within The Veil and thus able to sense and see things that are close to the other side of Veil or even partially intersected into our material universe.
UFO's as Visitors From Other Dimensions
This is referred to by some ufologists like Jacques Vallée and Dr. Josef Allen Hynek as the inter-dimensional hypothesis which instead of having UFO's as extraterrestrial visitors consider aliens to be from a different reality or dimension that coexist alongside our own reality. Dr. Hynek was one of the key researchers for the US government into the UFO phenomena under project Sign (1947-1949), project Grudge (1949-1952) and Project Blue book (1952-1969).