It can't be proven.... There is no actual footage of Bigfoot. Believe what you will. There could have been before the flood.
Genesis 2:4 ¶ There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.
5 And GOD saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually.
Giants that could be like Bigfoot. And legends like that of Pan the half goat guy could also originated from from before the flood. Of course you have to believe in the Bible and that is totally up to you. I do and before the flood the world was a totally different place........
I remember hearing somewhere that the giants the Bible refers to could have been angels (or angels that left God to follow Lucifer) that thought the early woman were beautiful and came to earth. The children that had angels (or would they be the fallen angels?) as fathers could be those giants. Perhaps that what Goliath was. - I'm not a Bible scholar, this is only an assumption. Don't hold me to that
Although I did kind of get off topic... Whoops!