[RoosterRedux]

Either demons or fallen angels or some combination thereof.

A lot of what I have read refers to demons as the spirits of dead Nephilim (i.e. halfbreeds) whereas fallen angels are, well, just that...spirit beings from another dimension who are anti-God.

[Hebrews 11:6]

Nephilim's spirits had not occurred to me as being part of the greater demon population, but you may be right. Sort of like fallen angels once-removed. However, Jude 6 [see also Luke 8:31] suggests they are no longer at liberty to be cavorting about masquerading as UFOs:

"And the angels who did not keep their positions of authority but abandoned their proper dwelling—these he has kept in darkness [zoφon], bound with everlasting chains for judgment on the great Day."