Most people, when considering the possibilities as to what UFOs might actually be, tend to lean heavily toward the idea of advanced aircraft that are both intelligently designed and controlled. Many reports do indeed appear to indicate that the objects seen appear mechanical or otherwise "manufactured," although there are also the occasional reports of rather amorphous "blobs" that seem to have little definite shape or form. Though the range of potentials for how an alien technology might appear to us remains very broad, perhaps there are other possible identities we could attribute to at least some UFO reports.
One unique (though perhaps unlikely) theory proposed a number of times over the years deals with the idea of "atmospheric lifeforms," as proposed by the likes of Charles Fort, Trevor James Constable, and Ivan Sanderson, among others. Essentially, this approach to various reports of UFOs supposes that some of the sightings might describe strange, atmospheric "beasts" that are essentially self-contained, energetic creatures.
Though the multitude of encounters detailing a more technological origin for UFO craft seems to leave little room for any credence to such ideas as "atmospheric beasts," perhaps engaging in a sort of "though game" with the subject could provide insights or some merit, by virtue of considering airborne anomalies from a less conventional perspective.
Generally, the idea of an atmospheric beast in the present context would involve a creature which either weighs less than air in a given atmosphere, or which may be capable of flying by some other means of wingless propulsion. With regard to research into the unexplained, one of the pioneering ufologists to take this approach with the study of unidentified flying craft was Trevor James Constable, a writer and researcher who outlined his ideas in the 1975 book The Cosmic Pulse of Life (somewhat sadly, I interviewed Constable a few years ago, and upon mentioning that I had obtained a copy of this book, he joked with me, "so you were the one who bought a copy!") Constable's idea of "critters" (photographs of which can be seen by clicking here) dealt with amoeba-like creatures that existed in Earth's upper atmosphere, and which occasionally fed on livestock and other creatures, hence seeking to explain some reports of animal mutilations that occur in conjunction with UFO sightings.
As strange as such an idea may sound at first, even Carl Sagan had discussed balloon-like aliens that might be capable of existing in the atmospheres of distant gas giants the likes of Jupiter. Popular fiction writers like Sir Arthur Conan Doyle and Ray Bradbury have dealt with such creatures as well, but since the majority of these sources for the lore surrounding jellyfish-like or amoebic creatures drifting around in the sky stems from either fiction or speculative science, many would argue that more fringe notions of the existence of such creatures were actually inspired by such works.
Nonetheless, during a conversation with a fellow researcher I had a few years ago, he divulged that during a conversation with a NASA astronaut who had worked with the space program years ago (who also asked to remain anonymous), his contact mentioned projects where he and others had observed strange "energetic forms" while monitoring spacewalks using visual apparatus that heightened visibility in the infrared range. Though my contact said the astronaut would not refer to the objects as "UFOs", he did mention that questions had arisen over whether they might be some form of energetic life. Furthermore, studies performed by NASA have also suggested the notion that plasma-based lifeforms might even come to exist in the vacuum of space.
If indeed "energetic life" were to exist, and if reports of glowing, amorphous blob-like UFOs were indicative of their presence, what might allow these creatures to produce such self-maintained illumination? Also, if they were consumers (as Constable proposed), it would have to stand to reason that they also produced waste. If not doing so in the conventional biological sense, could they perhaps do so through the release of heat or radiation? Radiation sickness is a common side effect afflicting those who have encountered a UFO, especially those which appear to be craft of intelligent design. It would be interesting to consider whether similar circumstances surrounded people's encounters with the less-tangible varieties of UFOs, also.
Again, while the supposition that UFO reports could be attributed to amorphous atmospheric beasts is unlikely at best, sometimes considering outlandish alternatives allows us to free our minds up to thinking in less conventional ways, also, and potentially even coming to unique realizations that contrast otherwise widely accepted views on certain phenomenon. What are your thoughts about atmospheric life, and could such species exist in places around the globe... or even in other parts of the universe?