Six ft fencing is the State law for stallions here. I am sure the law was written back when horses were used on farms and local transportation. Stallions were often dangerous on the farm, right up there with dairy bulls who regularly killed people before artificial insemination came along.
If stallion horses are getting out, call Animal Control. You "technically " should not need to fence to prevent a stallion getting at them. But I can see why you would!! Call and ask about local laws for loose horses/stallions so you know what you are working with.
I guess I would go visit said neighbors and warn them about preventing stallions from getting loose. You WILL be sending them bills for fence repairs, Vet costs on mares if he gets in with them and tears someone up. . Got any geldings? Stallion may REALLY go after it, causing a lot of damage to horse and fences.
Not YOUR job to catch stallion for his return home. Being neighborly is good, but loose stallion WILL create bad situations if it continues. As with bad, loose neighbor dogs, you have no evidence without reporting the occasions to show a continuing pattern of negligence. If the bad time does happen, you have legal grounds to collect for damages.
Sorry but I have a VERY LOW tolerance for loose animals of any species. People need to have facilities to contain their pets and livestock. I return the favor by keeping my pets and livestock at home with gates closed. Loose ONCE? Here’s your sign!! Fix things so it does not happen again.
I would suggest using the white electric tape or white coated wire for visibility, as the hot topper for the six ft fences. Aged galvanized wire is pretty invisible anyway, loose horse would not know your fenceline to see hot wire before he runs over or thru it to reach the mares.