A theory popular amongst Bigfoot 'believers' (especially Christians) is that  Sasquatch is a result of the Fallen Angels experimenting with genetics and/or mating with human women. 

This theory is also supported by many in the 'UFO camp', explaining the Fallen Angels as the terms used to describe an alien race.

It stems from Genesis : "When man began to multiply on the face of the land and daughters were born to them, the sons of God saw that the daughters of man were attractive. And they took as their wives any they chose. Then the Lord said, “My Spirit shall not abide in man forever, for he is flesh: his days shall be 120 years.” The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of man and they bore children to them. These were the mighty men who were of old, the men of renown. The Lord saw that the wickedness of man was great in the earth, and that every intention of the thoughts of his heart was only evil continually."

 in The book of Jubilees

Chapter 5.

1

: And it came to pass when the children of men began to multiply on the face of the earth and daughters were born unto them, that the angels of God saw them on a certain year of this jubilee, that they were beautiful to look upon; and they took themselves wives of all whom they chose, and they bare unto them sons and they were Giants.

Chapter 7.

21 from fornication and uncleanness and all iniquity. For owing to these three things came the flood upon the earth, namely, owing to the fornication wherein the Watchers against the law of their ordinances went a whoring after the daughters of men, and took themselves wives of all which they chose: and they made the beginning of uncleanness.

22 And they begat sons the Naphidim, and they were all unlike, and they devoured one another: and the Giants slew the Naphil, and the Naphil slew the Eljo, and the Eljo mankind, and one man another.

And the Book Of Enoch

In the book of Enoch it is told that in ancient times, 200 heavenly Watchers rebelled against God in heaven. Led by the angels Semyaza and Azazel, the Watchers came to Earth on Mount Hermon, where they mated with human women. This resulted in the birth of bloodthirsty hybrid giants and later led to the Great Flood.

Chapter 6

1 And it came to pass when the children of men had multiplied that in those days were born unto them beautiful and comely daughters.

2 And the angels, the children of the heaven, saw and lusted after them, and said to one another: ’Come, let us choose us wives from among the children of men and beget us children

Chapter 7

1 And all the others together with them took unto themselves wives, and each chose for himself one, and they began to go in unto them and to defile themselves with them, and they taught them charms and enchantments, and the cutting of roots, and made them acquainted with plants