
Do you have any friendly horses near by?
What i have done with my three dogs (one of whom i have SEEN chase every kind of livestock u can imagine) is to introduce them to 'dog proof' horses.
I would never say that any of mine are 100% reliable around livestock, but what animal is? But, they now show no interest in barking/chasing/sniffing at the heels of horses!
Now i can pretty much trust them around loose or tethered horses (and unfortunately for me, no i cannot keep them on a lead in case of loose/tethered horses, as they are on rough ground/council land, are not meant to be there, and are moved around every night, so i never know where they will be, and im d*mned if im keeping all my dogs on a lead cos some cheeky b*gger doesnt want to pay for grazing!).
I certainly do not trust them around ridden horses though, the temptation is too great, with the odd spectacle of a person sitting on a horse.
See if you can show your dog that horses are just big animals that want to sniff at them. If that works, it soon takes away the curiosity value.
Failing that, i would say using a remote spray collar would be a good option, with a lot of dogs, the urge is there to go behind an animal and move it on by nipping. I have seen a previous dog of mine chase a horse and get kicked, and it was the most horrible experience of my life!
Em