Have you ever noticed how closely sketches of ETs resemble demons?
Even if we limit ourselves to Euro-Christian mythology and folklore, the forms demons supposedly take are so varied - including but not limited to, common animals, specific people relevant to the observer, intangible phenomena like flames or shadows, utterly bizarre mix-and-matches of limbs and bodies, and even no form at all -, as well as the fact that demons do not demonstrably exist saying that something resembles demons is meaningless. Now, if you are talking about the contemporary stereotypical depictions of demons and contemporary stereotypical depictions of ufological aliens… imageimage…No, they do not resemble each other at all.
Have you ever noticed how many "alien abductees" describe their experience as demonic and their captors as looking like demons?
The brain at times generates erraneous stimuli that do not get filtered out and/or goes into altered states of consciousness where it loses track of time and/or external reality. And yes, this can happen to completely functional neurotypical people and without the need of drugs or deliberate mental induction. Some of these mental phenomena can include highly unpleasant experiences, such as sleep paralysis.
Higher functions of the mind and retrospective attempts at recall attempt to interprete these stray signals, resulting in confabulation - and that confabulation draws from the cultural ideas of the patient.
With the Scientific Revolution, industrialisation and the colonialist self-image of an enlightened civilisation so superior to the primitives it subjugated (for their own good, of course), most people started to realise how silly demons and fairies are. And yet, Man as ever remains a far less rational creature than he wants himself to believe, and people still experienced strange sensations such as missing times and waking up at night, unable to move and constricted in the chest, with strange visuals in the periphery and painful sensations in their nether regions. And so, as astronomy advanced and a new understanding of our world being part of a universe so much bigger emerged, many of the old fantasies, and old superstitious explanations for weird occurrences were transplanted to the stars, starting with the planets of our own system, and then to prominent stars such as the Pleiades.
TLDR:
Any resemblance between aliens and folkloric fairies and demons come from them all being incorrect interpretations of the same psychological phenomena.
Another thing to consider: Of course, UFOs are a phenomenon most of all of the Modern Era. Urbanisation, with light pollution, indoor work and time that passes by the calendar rather than the seasons has led to loss of familiarity with the night sky, and at the same time, Man finally realised his longing to invent machines that would allow him to fly (not coincidentally, the first UFO waves in the late 19th Century reported cigar-shaped objects - you know, like zeppelins…).
Could we be in the beginning stages of a fake alien invasion?
Could it be a genuine alien invasion, where the invaders have spend time studying our cultures in preparation and thus have decided to exploit our religions and superstitions?