I like this idea is well! The trick, though, is that the Cowboy is more like an irregular unit than a regular military unit. First of all, they should be able to Build Pastures (their original job, right?), and if they Fortify in a Pasture, you get +1 Food and +1 Coin from it. Second, they should cause one for each one that occupies a city (they're rural folks and tend to make the city slickers uneasy). I'm not sure how easy or hard it would be to do all of the above, though.
Other than that, they should be a little less powerful than Cavalry (since they're not nearly as disciplined), so I'd give them the following stats: 12 Strength, 2 Movement, cost 80 Hammers; Does not receive any defensive bonuses; 1 First Strike; Has 60% chance to withdraw from combat (a lot more likely to flee from combat than Cavalry); +25% Attack Versus Cannon (again, not as disciplined in combat); Can upgrade to Cavalry (but you should be able to keep making them up through the Modern era; they never become totally obsolete because of their Pasture benefits).