This is a Metal Gear Solid 3 achievement.
MGS3 achievement. Every area in the game has a hidden Kerotan. You need to find all of them and shoot them. They will make a funny noise when you hit them. Do this to all 64 of them and you’ll get your achievement. Some of them are missable, so if you ever feel like you've passed an area you won't be able to return to and didn't find the Kerotan, don't overwrite your save. Load your last save and try to find the Kerotan. This applies to the bike chase near the ending, for example.
Kerotan shooting is cumulative, so if you miss any on your first playthrough, you can shoot it on your second playthrough and still get the achievement. Just take note of the ones you missed, because the game doesn't provide a way to keep track of this.
For videos of every location, refer to this thread.