You will start finding animals that count towards this achievement very early on. They spawn frequently and you can revisit areas later that have them. The below list are the types of animals that count towards this achievement. Just remember that after you kill them, you must go loot them or it will not count. This can be tough sometimes with birds since they can be in spots you cannot reach. You can kill them while they are on the ground or flying, and with any weapon. To save ammo, you can kill them with Rope Pull when you learn it.