Red Dead Redemption

This achievement is called
"Manifest Destiny"
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)

ADDED -
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.

This achievement is called
"Manifest Destiny"
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)

ADDED -
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.

There are 20 buffalo that don't respawn so once you kill them they are dead.. mostly found in the great plains. I personally tried to complete this achievement before completeting the mission "For Purely Scientific Purposes" and i managed to kill 18 buffalo but couldn't find anymore even using bait .. so i gave up and continued with the story .. when i got to the mission "For Purely Scientific Purposes" at the beggining you are riding with mc and and nasada . You come across what i guess is supposed to be a heard of buffalo they comment on it.. but in my case there was only two running i popped them both that was it.

Hope this helps. in short you need to complete that mission before you have access to all the buffalo

Credit foes to ShadowWolf810
Steps to Manifest Destiny Achievement

-Get access to the northern part of your map known as the "Great Plains." You will eventually gain access to that region by playing the game!

-Go to the buffalo picture on the map at around 1 am. A huge herd of buffalo should be peacefully grazing there.

-Sneak up quietly and try to take out as many as you can before you spook the herd and they start to stampede.

-Kill as many as you can, you can still come back afterward if you don't get them all and repeat the process.

-Once you've killed all the buffalo, you'll get the achievement.

The best way to get the achievement is being Master Hunter Level 5. When you are level 5 you can buy the buffalo rifle in Blackwater. With this rifle the buffalo's will be dead in one shot.

You might want to come back a few times because it is hard to get them all at once.

The Buffalo's won't respawn.

This achievement can be unlocked once the player gets to Blackwater. There will be a herd of 20 buffalo wandering the Great Plains area (there is a picture of buffalo on the map in this area). The player must kill all 20 buffalo in the herd to get this achievement.

Since the buffalo run when attacked or if the player approaches too closely, attempt to kill them from a distance, using head shots from a sniper rifle. Done right, few of the buffalo, if any, will run. If they do run, do NOT chase them. Simply leave and return at a later time (1-2 days in game) and repeat the process until all of them are dead. Giving chase could force one or more of them off of a cliff, or into the water, or even onto the train tracks in front of a train. If this happens, the dead buffalo will not re-spawn, and the player will not be able to kill it, making the achievement impossible to unlock in single player until a new game is started.

In this unfortunate situation, there is a way to get the achievement in multiplayer. If you can find someone who has reached Legendary Rank 3, Level 35 in multiplayer (no mean feat, since getting each Legendary Rank requires a player to reset at Level 50 and level up all over again), they will have unlocked the buffalo mount. If this person can meet you in game, they can call for the mount and you can kill it. Do this 20 times and the achievement is yours. The whole process should take no more than about 15 minutes. As far as I know, there are no buffalo otherwise available in multiplayer.

The way that I did this was to approach the herd by horse, take the Evans Repeater and use some Moonshine and use Dead-Eye to paint all the outside buffalo. After this, go straight back into DeadEye so your gun gets reloaded automatically, and shoot the rest. Do this one more time and then chase the rest of the buffaloes.

It made it pretty simple and I managed to take out the last 15 I needed at the same time.

Easiest way - get a friend who has unlocked the Buffalo online. Go into a free roam session and get your friend to call in the buffalo and you kill it. Repeat until you unlock achievement.