The Illusion Theory

According to such theories, the creatures are merely illusions : geological holographs, optical illusions comparable to the famous Brocken spectre, visitor-induced energy trace image, electromagnetic phantom, or hallucination engendered by oxygen deficiency.
But Bigfoot has been seen in such a diversity of countries, climates and situations by so many witnesses, sometimes collectively that such a theory is almost invalid.

The Ancient Race Theory

Another theory is that the Yetis are descendant of the A-o-re, an ancient people that fled into the mountains to escape their enemies. In the following millennia, they degraded to a race of monstrous creatures. Victims of genetical decay, only a few remains hidden in the nature sanctuaries.

The Alien Theory

Other theorists associate sightings of these creatures with the activity of extraterrestrial beings. They notice that the presence of Bigfoot is often followed or preceded by paranormal phenomenon. The mountains where the beast appear could possibly be gates to other dimensions and alternate worlds and the yetis guardians who deter would-be intruders or trespassers.

The Demonic Theory

According to this theory mostly found in Tibet but also in the indian tribes of America, Bigfoot is a demon (not necessarily evil) that comes down on Earth. He has the same abilities as those sometimes mentionned as belonging to the great adepts of the Tantric way : resistance to cold temperatures, ability to move at high speed on large distances, levitation, telepathy and mind control . This theory goes with the one that pretends that Yetis are Shamen that shapeshifts into monstrous creatures.