Rescue the kidnapped sheep from the soldiers in Wialo Village.
Metal Gear Solid 5: The Phantom Pain - Extract the Little Lost Sheep
It's probably best not to ask why the soldiers at this outpost have stolen a sheep and are keeping it under a two-man guard.
You might struggle to find the sheep when you get to Wialo Village, but if you access your iDroid and select the mission again, the sheep will pop up on your map.
Take out the soldiers guarding the sheep, knock it out with a tranquiliser, and fulton it out of there to complete the mission.