What about Upright walking Hominids such as Sasquatch or Yeti, or even
Old Yellow-top?
In the last 11 years, researchers have discovered
at least 25 new species of primates. Many of them have been discovered in some relatively small ecological areas - areas bordered or threatened by vast tracts of civilization, such as the Asian subcontinent, especially Borneo.
Many of the recently discovered mammal species probably remained elusive for such a long time not because of their intelligence, but because of their nocturnal behaviour. Scarcity in numbers is often a factor; size also (the biggest weakness of the "old yetiquatch" argument.)
Borneo, while thick with jungle, is also relatively small. There are other, much vaster and remote forests in the world. Many of these forests have native legends of upright walking beasts. The sasquatch stories can be found in pre-columbian native lore; the Yeti story is also old; and yellow-top was known to the natives before the miners in cobalt Ontario described their encounter.
Also, here's a cool link describing the recent
discoveries of new species in the USA.
So when it comes to old stories of an upright walking ape... if it took until 2004 to discover the
Arunachal Macaque (a relatively large primate found in North-East India, one of the most densely populated areas in the world) then I'm open to the possibility of there being some undiscovered larger primates in some of the more remote regions of the world... say Nepal, B.C or Alaska.