The Final Station

To be able to get this achievement you will need to find in the game the infected horse and shoot it with your pistol or get close to it and use B.

The horse is located in chapter 6, here is the starting point :

Go right and you will find it here in the stable :

Shoot the horse in the head and you get your achievement .

EDIT: Kiasano comment that ''I just punched it and got the achievement. So if you wanna save a bullet, moving up and pressing 'B' does the trick, too.'' so press B if want to save a bullet

Despite the name of the achievement, you don't actually have to shoot the horse in Level 6 of the DLC with your pistol. Simply whacking it in the head with your button will down the horse, and it will count for this achievement.

If you do decide to shoot it, feel free to use Restart Level to get your bullet back; the horse is near the very start of the level, so it shouldn't take much effort to get there.

This achievement requires you to shoot a horse in the DLC.

Once you get to the 6th level, you'll be on a farm. From your car, head into the building to your right. Pass through this room and go through the next door on your right and you'll enter some stables. You'll see a black head with glowing white eyes peeking above the wooden fences in the background. Simply aim at the head and shoot it and you'll get the achievement.

If you somehow don't have any pistol ammo no need to worry as you'll get another 11 attempts if you're going for all the achievements...