I recently found a video on YouTube, that I think can be used as credible evidence proving Bigfoots existence. The video depicts a guy in Back O' Beyond shooting at Bigfoot. Unfortunately, the video was filmed poorly on a phone, however you can clearly see Bigfoot at 0:28 and 0:42. Unlike many mods out there, Bigfoot didn't attack the player but instead was running away. And considering this was found on the PS2 version of GTA SA, I don't think this is a result of mods.