Walkthrough for Birth of the Conservation Movement of Red Dead Redemption: Undead Nightmare.
1. Go to the green question mark in Tanner's Ranch under the Tall Trees.
2. Your mission is to kill six Sasquatch, which is a name given to Bigfoots in RDR1.
3. Go to the green areas around the house to find them.
4. Every time you kill one Sasquatch another green area appears around the house.
5. Location of the sixth Sasquatch appears as a green question mark and when you get to him it's a different story.
6. You can kill him or let him live as the last of his kind.
Tip: There is few posts in reddit saying if you don't kill him, you can see more of his kind around this area later.