Well, who knows. Gorillas were considered mythical until the Victorian Age. And people thought the platypus was a fake even after specimens turned up. (The poor platypus does look fake. What a weirdo.)
http://www.animalplanet.com/tv-shows/monster-week/mythical-animals-that-turned-out-to-be-real/
One interesting argument for “real” Bigfoot is the distribution of footprint sizes. I read at a couple of sources that someone gathered all the known sizes of footprint castings and graphed them by size.
If the footprints were all concocted you would expect a Gaussian distribution with a single hump at some large size. Instead the distribution showed two humps, which you would normally get from graphing the foot sizes of males and females in a real population.
Of course I can’t vouch for whoever did the graphing, but it’s interesting in any case.