Sound barrier walls are not very effective at reducing noise because sound waves go over them. I would recommend a very dense and tall hedge or even thick strip of woodland to absorb sound waves. Good diagram here showing sound waves.
I’ve also boarded horses at a place where dog boarding was done and the 24/7 barking of distressed dogs is annoying for sure. The boarded horses totally tuned it out very quickly. I think once your resident horses get used to it, any new horses coming in will quickly pick up on their cues that it’s no big deal.
If you can rearrange your property to put the horses further away from the dogs though… so much the better.