There are no horse spawners in buydebug. Unfortunately, it doesn't look like there's a way. Nobody has made a mod yet that would override it that I can find either. That doesn't mean you can't have horses in Bridgeport, you just won't see herds of wild horses running around. Nor will you see deer or raccoons.
I have been using Riverview, and I used to see the wild horses, but then again I'm not playing as excessively as I used to, but there would be a herd there like the one in Appaloosa. I'm currently also using the original town that came with the base game (I can't remember, sunset or somthing?) But I was just on and off to make my horses and take some pics. There are a couple things you can try:
There are gnomes in the animal section of the buying tabs that will attract wild-life, stray dogs, and stray cats, or tell them to stay away from your property XD In which case they will defecate next to the sign and leave. Anyway, you can place a wild-life gnome and set it to attract wild-life. Be warned! This will also attract scavenging raccoon and hopping deer. I would ASSUME it attracts WILD horses, since they are WILD, lol.
You can also buy the 'popular' life-time reward if you have a horse, I can't remember it, but something along the lines of they will be the most popular horse in the herd. I never used it because I didn't want wild horses being attracted to my horses and flood my small pastures XD
Good luck. Also, they tend to run through majority of the parks in towns all day, so you could try building a small park with some welcoming horse items to lure them (i.e. water troughs, the hitching posts so they have hey and water.)
I'm not sure if this will work, cause I only have the three towns, but good luck!
This message was edited 2 times. Last update was at 05/26/12 12:15 AM
Riverview is a suburban area. Bridgeport (which came with Late Night) does not spawn wild horses at all. Just like it would be a little weird to see horses running down the streets of New York City, you won't see them running in Bridgeport. Any world that has the same world icon as Bridgeport won't spawn wild horses.