My family and I live in a rather remote location. It makes for a long commute, but does give us the opportunity to observe nature in a way that most people never can. Since moving here, I've personally seen everything from earthworms as big around as a pencil to foxes to bobcats to eagles to coyotes. The most impressive sighting I've yet had though happened on my way to work the other day when I saw my first black bear.
I was coming around a bed in the road (slowly, thankfully) and first saw only a fleeting shadow moving onto the road. As a came around, the bear was most of the way across. I slowed almost to a stop, while he (or she) skidded to a halt, nearly fell down, and then "gallumped" back across the road into the brush.
To begin with, I must say that anyone who talks about "itty, bitty" black bears has either never seen one or must have grown up around grizzlies and kodiaks. This was not a dainty creature by any stretch of the imagination. He probably weighed more than me, and that's saying something.
More importantly for present purposes, I was struck by two other facts.

Second, its gait wasn't what I expected. To be honest, I'm not really sure what I was looking for, but the long, reaching, loping pace looked odd. It almost looked like a man trying to scramble along on four legs, at least at first.
Which brings me to my point. Aside from another interesting animal tidbit, this little encounter made it easy for me to see how, in certain circumstances, someone could mistake a bear for a bigfoot.
At first glance, the shape can seem more human-like that bear-like (if one is expecting teddy bears instead of long legs on a lean body). The loping gait can almost look ape-like, and of course the matted black fur would fit perfectly. if I hadn't seen his face very clearly, and I was an ardent believer, I might indeed jump to an unjustified conclusion. The picture below is an example. While some people vehemently believe that it must be a "juvenile bigfoot", in the absence of a clear look at the face, I must agree with the fish and game department: it's a black bear with mange. Not terribly exciting, but probably true.

Of course, I'm not implying that bears can account for all bigfoot sightings, that anyone who has a clear line of observation (as I did) could make such a mistake, or that anyone sufficiently woodcrafty could be fooled. But I do think that at least some sightings can be blamed on this source.
I do think it interesting nonetheless.