This achievement is called
Kill the last buffalo on the great plains in single player
Yes pretty sad achievement, but on to info.
Number of Buffalo = 20
Location Great plains, if you hit the farm or tree lines you have gone too far.
They seem to appear in the middle of 4 different areas if you are looking for them;
The lower case h in beecher;
around r in great plain;
the middle buffalo on the map (close to blackwater);
close to wreck of serendipty (where coyote or fox pic is on the map - southern part.)
(if anyone finds any other locations i will update and give credit)
Jonnyb0b stated that he has
"I've seen buffalo northwest of Blackwater in the little corner the river makes with the open water a couple of times, don't know if that is enough to qualify it for the list."
You only need to kill them you dont need to skin.
Range with a sniper is best as you can get the furthest ones away and then the closest ones. If any of them get out of your line of site you are going to go hunt for them.
I also suggest if you loss site of them use your campfire to savegame, nothing worse than being pounced.
You could also save once in a while, while searching for them, once you find them if you miss some, you could reload and try again, or just go searching.
Rastabikerman - This player has mentioned that he got this achievement by killing someone's buffalo mount from within multiplayer. This is the first time i had heard of this happening so if someone could confirm that would be awesome
(Even though this says single player it seems quite a few people are unlocking it in MP)
Leon Kowalski - This player said the following and 2 others have confirmed it worked for them.
"For Purely Scientific Purposes" mission mentioned above. I rode all the way out with Nastas and the Professor to Bear Claw Camp, and never saw any buffalo nor did anyone comment on any, as may have occurred for Fire Hawk D. The only encounter was the scripted one with the bear. So once I got to the camp, I just kept riding, and once I got too far from Nastas, I "failed" the mission, and cancelled out. Then once I was back as Jack, I rode to the big buffalo drawing on the map and there was a herd of at least ten buffalo there; certainly more than the five that should only have been left. Maybe by (partially) replaying an old mission it respawned the herd. I did not try to reproduce this after burning so much time riding around Great Plains and encountering a ridiculous number of people and very few buffalo, but maybe it will help the next person.