Darkest Of Days

The first level works well. Simply run at a horse and hit RB until you hit one. It may take a few tries, but you should get it rather quickly.

Such a hilarious achievement that is rather easy to get. The first level you'll be forced to play is at the Battle of Little Big Horn. There are native americans on horses. Go up to one of these horses that is running around and punch it. Even though the achievement text says "in the face", I actually died a plethora of times trying to literally punch it in the face. Instead just punch it wherever and it will pretty much go down.

If you missed this one on your playthrough, it's not a big deal since it's the first level.

Theoretically you could get it on other levels as well (there are more horses out there), but since this level is a tutorial of sorts, it's a bit easier.

This can be done on the very first playable level "Little Big Horn" where you have to kill the "injuns". Press RB to melee a horse. You can NOT melee a dead or friendly horse and have it count for this achievement, I tried lol. Also, you do not have to melee a horse in the face. I hit a horse on its right side and the achievement popped.
Congrats, PETA now hates you lol


SPOILER

Right after the second save point (right where you pull your six shooter out) there will be like 5 or 5 Indians on horse back coming at you . Just pick one and press RB to melee the horse.