Kill the last buffalo in the Great Plains in Single Player.

This can only be completed in the Great Plains. You will first need to finish Act 2 to gain access to this area. Make your way to the Great Plains and ride around until you find the Buffalo. They are normally roaming around in the open. To unlock the achievement you need to kill all 20 of the herd. Make sure that you pick off the buffalo from a distance, if they run off don't chase them. Save often and come back to this area every once in a while to kill the odd buffalo you see. Once you kill the last buffalo the achievement will unlock.

Attempting this early on with a less powerful gun can take a while. If you have a sniper rifle use that instead it's a one shot kill on any buffalo.