Yes . it will get worse. so will the flies. Please do not complain , you moved there Next to them. The trend in this state, is people move next to horses, dairies, farms,ranches, ranchettes, Then after a wet winter, or dry hot summer, complain about dirt, flies, animals breeding , yes they do , right out there in the open, geldings airing out things, bugs, and more bugs and smells.
The people with the horses probably had the home built around them.
Please do not pet or feed the horses without the owner being present. Also if the owner puts fly mask on the horses, it does not blind them . I had to actually show a neighbor that the horses vision is not impaired, I took a new fly mask and put it on his face , of course he agreed to do this. Also on the manure, in my area it must be removed once per week. You need to speak with your neighbors if you have legit issues. And you will have mosquitoes , lots of them. Keep your dog fenced in your yard , and your dog will bark like crazy at the horses and keep your fence maintained, and do not spray weed killer near the horses.