I think what we are getting in the horses EP is kinda what they are able to fit into their budget. All EP's have kinda the same content, some maybe had a bit more content than others but they are kinda similar.
So I don't think we would have been able to get horses together with the content in another EP. If we would have gotten them in cottage living then they would had to take something else out, like the cows and lamas.
The thing is that sometimes they develop some things with great detail which works great in the game and sometimes they make things that are shallow and doesn't work that great and it seems to be very much depending on the game itself or just what they choose to focus on. For example the robotics skill and what you can make with it in Discover University is incredibly detailed and that's in an EP where the focus is on your sim going to university.
So I think it's difficult to say for now if horses could have been included in another pack or not, it depends how well they will work. They could be fancy bicycles that sims will ride into their houses and glitch and cause lag in the new world with their open and limited areas or they could be actual horses with loads of features that will work well together in all worlds and really expand the game.
