@ayrabz The dismay you’re feeling about getting new neighbors close to your house is totally understandable. But you need to accept that this is an unavoidable fact of horsekeeping on a small lot. Everything you’re saying about them (they’ll be right in your space because their lot is not all that suitable for how they’re using it). could be said about you, even though you got their first. So in the interest of a healthy neighbor relationship, maybe try to reframe your thinking: just as your family has made the best use of the land without intending to be bad neighbors, they’re probably going to do the same. Sounds like you believe you are grandfathered for having your facilities right on the property line, but I’d urge you to confirm that this is actually the case.
If it’s not clear that you’re truly, legally grandfathered, it’s all the more reason to stay on good footing with the newbies, so that your building (that generates odors and flies and sits RIGHT ON THEIR LINE) doesn’t become a source of complaints. For the fenceline-- that’s not an immovable, permanent thing. If privacy is a big concern for you, move that line of posts in a little bit, and put up privacy fence. Or why not just add privacy boards to the inside of your existing fenceline, with some hotwire to keep your horses from messing with it.
Sorry, I know none of this is what you want to hear. This least of all (and I say this just about every time these MY NEIGHBOR’S TOO CLOSE threads come up): if you <general you> don’t want to see or hear neighbors, buy more land. Everyone sputters in response about why this is impractical / impossible / whatever, but it actually IS possible in many areas of the country. So either decide that this level of privacy is a top priority and arrange your life to fulfill that priority, or acknowledge that you have other priorities and therefore need to accommodate some sacrifices on this one. Bottom line, the neighbors have every right to use their property however they want, within the bounds of the ordinances/law. It is not their fault that the amount of land you own does not meet your needs for privacy, so any anger directed their way about it is entirely misplaced.