Well in regards to UFOs, first off, if I were an alien life I would not want to visit me... we are a terrible species right now. But my theory is that it is possible to make a UFO with basic technology.
In all truth though a UFO can literally be anything and be accurate. UFO is Unidentified Flying Object. So even a Frisbee counts, which 90% of photo "evidence" looks like. Also, the government did the same thing with the stealth bomber, the original UFO I believe. It's the perfect way to hide a secret weapon.

I believe it is possible. I mean, I don't believe that it was aliens, and I will believe such until the day I meet one. Like what you saw. There is no proof it was aliens, but you did see a UFO. What that UFO was is the real question. Eventually if I ever end up cracking the one last piece of technology I need for creating a functioning "flying saucer" replica I'll probably end up posting it. (Yeah nerdy I know)
The ones I don't believe one bit is the absolutely massive saucer that followed a plane. His radar said it was something like the size of a small city. There is no way that wouldn't be seen by satellite.

Well whatever it is animals sure are in tune with it, and I've noticed that they scare them a LOT.