Lately I've been torn between two games, both concern themselves heavily with horses. After watching the quick look I was almost immediately hooked on Mount and Blade. I downloaded and loaded it up... that game is awesome. Its ugly as sin but it only adds to the charm, that game is so deep you get sucked in. I love the diplomacy, the battle, the economy, everything. The settings is great, plus the re-playability... I don't even want to think about the amount of time it could suck from my life if I didn't have a correct hierarchy of responsibilities. 
The second game I've gotten hooked on is Red Dead Redemption. Now, I've not been able to play it as much as I would like, what with work and girlfriend, but what I have played has been more than enjoyable. There is so much to do, sometimes maybe too much... I have been thoroughly distracted from the story, which may have gotten lost on me. What with the whole hunting, strangers, treasures, challenges and everything... But other than the fact that I cannot help myself from being a completionist the game is amazing. 
Now I don't know whether its the settings that make it great or the fact that the only means of transportation in both games is horses... This has been weighing heavy on my mind lately, and I think the question is more of a chicken and the egg. The horses make the setting great and the setting makes it possible to have horses. Either way I'm not going to start playing My Little Pony Adventures anytime soon.