Red Dead Redemption 2 features so many secrets unrelated to the main story that it can be difficult to track them all. From vampires killing people in Saint Denis to ghosts and even UFOs, the game always knows how to surprise players.

To find the Big Remains, as the game calls the remains of Sasquatch Bigfoot, you need to climb Mount Shann. The mountain is found right below the "S" in West Elizabeth on the map.
Once at the top of the mountain, you need to spot the area with an eagle on its edge. Head to the left of it to find a path hidden by plants, and proceed through it to find a small cave. Inside the cave, will be an unusually big skeleton.
Arthur will make a sketch of it in his journal. The journal's description - “Found a massive skeleton. Wonder how big the man was, or if he was a man at all.” - leaves little room to doubts about to whom the skeleton belonged to.
Once you have found the giant skeleton, you will have done everything you needed to discover the Giant Remains Easter Egg.