I for one believe there are unexplained objects in the sky people are seeing, and have seen for centuries. Some very old oil paintings have renderings of UFO objects in their background sky. Where would artists of centuries ago get the idea to render such things in the sky in their paintings if someone hadn't seen the same thing those today are seeing?
I also believe the UFOs are not aliens, but the angels, popping in and out of the heavenly dimension which exists as a veil behind our earthly dimension. Ezekiel 1 is very revealing in the Hebrew, showing what Ezekiel really saw was a type of round craft turned on its side, and was the color of highly polished bronze. God's Throne was on it. So that would mean not all sightings are of bad angels.
Back in 1989, Christopher Walken played in a movie called Communion which was based on the true story of an incident that happened in a country home in New York state. When his wife got him to go to seek psychiatric help because of dreams he was having, the doctor did hypnosis. While under hypnosis he described his abduction and when he asked the supposed aliens to show who they were, they kept changing their appearance, and he said, "You're not going to show me who you really are, are you?" (I paraphrase).
There is no such thing as aliens or monsters in God's creation. But God also created the angels with the outward image likeness of Himself which is the image of man. The angels look like men (Genesis 19).
One of the things our Lord Jesus warned us about for the end is He said it would be just like the days of Noe (Noah - Matt.24:27). What was the main event at the time of Noah? Per Genesis 6, angels ("sons of God") left their own habitation and kept not their first estate (Jude 1) and took wives of the daughters of men. Their progeny were the Nephilim or giants, a hybrid humanoid race of men of great physical stature, and I think supernatural abilities, since Genesis 6 saying they were "men of renown" (men of the legend) is likely where the mythologies of gods in many ancient cultures came from.