Outdoor Sports Blog links to somebody who believes Big Foot is real.
All I can say is, most of the backcountry is unexplored by humans; judging from the rarity of seeing other people on the trails in populated areas, I could see how a species could go undetected for centuries in really remote forests and mountain ranges where there are no trails at all.
That is all the credence I’ll give to Big Foot, though. It’s like the Loch Ness Monster: we don’t really want to find Nessy, it would ruin the mystery.