I have always been fascinated with this topic as well. Can't recall whether it was the Cayce readings, but there were some sources that hinted these are the remnants of the genetic experiments and hybrids created during the time of Altantis. Bigfoot were designed to be 'beasts of burden' and move heavy equipment, but were controlled utilizing sound frequencies.
Other sources have indicated these are beings from the exploded Maldek, who having destroyed their own planet, were being forced as a collective to reincarnate and live in closer harmony to the planet using 'sticks and stones' rather than large-scale weapons- hence their primitive form.
More recently, I've heard the likes of Ed Dames and others describe them as 'avatars' for certain highly developed and interdimensional species. They can utilize the body of the bigfoot to incarnate and experience the physicality earth has to offer, while being able to observe as needed.
Possibly all three have some validity, or maybe they just make interesting stories- no way to know for certain, but it would be interesting to have some more insight