I think it would depend on the horses...but if it's something extremely loud I would say it would startle them not to mention it might do a number on their hearing.
I would find a reputable place to purchase and install the muffler. I actually just got a new one on my truck a few weeks ago. A professional and reliable place knows about all of its products. The guy we use knows how all of his mufflers sound and which ones go best with what vehicle and customer. My new muffler makes my truck sound good as well as giving it a little extra horsepower. It's not very loud but it is deep and throaty.
So in short, find an experienced seller. :)
I actually have a video haha. I have changed the muffler since then but it's pretty similar and you can tell the difference. The first part is the factory muffler and the second part is the new one.