I have seen some of the wicked things that DARPA has invented. They have a suit that mocks the surroundings of the person wearing it. This makes them invisible sort of, since you do not see them standing there.
Some of the storys of Bigfoot has him disappearing into something or morphing.I have often wondered how many sightings are military in nature, possibly soldiers in a suit such as this.
Any way, the cloaking technology continues to advance. I often get the strongest feeling that the Bigfoot are some how very super intelegent. It is possible that a super intellegence roams the woods in bear furs that are actually some type of color changing suit that mocks a bears fur even.
As far as the possibility that the BF are aliens, it is just a theory. But who knows where the proof lies.
Here is a page I found that puts BF in the vacinity of UFO's. The Bigfoot I saw flying looked exactly like the one depicted on this page that is a 1967 film. (at least my memory says it was 1967). The face was exact!