COWBOY it's a singleplayer custom mission for Call of Juarez: Bound in Blood made by vodka-kyo.
In this custom mission, you're playing as Thomas and you need to complete various tasks in order to join the Juarez's team. These tasks include shooting bottles, finding money bags or shooting down the barrels. This mission is proclaimed to be one of the most hardest missions back then, for regular COJ players.
Copy the .pak file from zip archive and paste it to C://Program Files/ Call of Juarez Bound in Blood / COJ 2 / Data / Mapsextra
Enjoy!