The Reddit contributor OscarTheSwede was cool enough to share how he found Bigfoot in GTA 5. Thank him, not me!
“So in the mission where Franklin are chasing after three guys that Trevor wants dead, they crash in a forest. Michael and Trevor arrives at the forest with a helicopter so Micheal can shoot them from the air with a sniper rifle.
So now to the easter egg. If you aim almost as far as possible to the right bottom corner you will see Bigfoot. Use the thermal to spot him and make sure that you don’t put your sights on him, because he will disappear then. If you want to get a good look at him, zoom in as far as possible and keep him in the corners of your sights. He won’t disappear then.”