Training not to chase horses and a good recall.

RRR should do just fine.  I've used it to train dogs to recall off running stock - BUT, the steps must be broken down even further than you would be inclined to think.

Approach it like you were training a dog not to react to kids on bikes or cars going by.  Here's the steps (and this will help the owner continue with the training since this will NOT get done in a week of training):

• On long line (for dog/horse/owner - lol poison ivy) in safely fenced area out of sight of horses, increase distance until dog recalls at max allowed by line without hesitation.  Any hesitation, merely go back and reinforce previous distance.
  
• Off line, start back close to feet, increase to maximum distance allowed by safely fenced area.  Again, treat refusals as a signal that the dog didn't truly get it at the previous distance.
• If possible, on line IN sight of horses, but with dogproof fence between them.  Go back to practice on line, from short to long distance (I use a 50' cord - cotton laundry line on a light snap bolt).  Again, treat refusals as a signal that the dog didn't truly get it at the previous distance.  Watch for the dog's FIRST signals that the horses have kindled interest in him.  Work from BEFORE that distance first, then at that distance, until the dog's responding in a relaxed way to you, then move to a distance where he's got "that look" again, and so on.  Use this as your guide for future steps - watching him for signs that he's stimulated, but not "in the zone" where there's no point trying.
  
• Now in the safely fenced area IN sight of horses, but off the line.  Start from short distance to long distance.  Again, treat refusals as a signal that the dog didn't truly get it at the previous distance. (Did I mention that?)
• Now on line in an area where the dog can choose to get to the horses.  Start with short distances and work up to long distances where the dog is working on the OPPOSITE side of the area from the horses, and towards you, standing near what he wants (the fenceline where the horses are).  Don't try to block him if he slips past and gets in the horse pasture.  Instead, go get him (grab the line) and return to  the last place he recalled reliably.
• Next try it when you are placed so that he has to come away from the horses to you.  All the same rules apply - short distance to long.  You'll find the dog is starting to get the "rules" and is responding better to the loss of freedom when it happens.  For most dogs the message is clear - you mess up, you lose some freedom.  You give to me, the boundaries get wider.
• Next is the scary hard part to do - letting him off line.  Try not to be tense, or let the owner be tense.  All the above should have been about the dog showing YOU that you can trust him - so do so!  Apparently poison ivy is the greatest danger here, so accept that possibility and go on.  Failure is the dog's signal, as usual, that he wasn't ready for that step forward.
• Follow the previous two steps, but off leash.  If he escapes into the horse pasture, don't chase him, GO TO THE HORSES.  That's where he's heading anyway.  If possible and safe, block his access to the horses (I use a verbal correction - NO- OFF!), then walk away and give him a chance to recall.  He might surprise you!  But don't act really happy at this point - he wasn't a good boy - otherwise he may start freelancing just for the chance to do the BIG BONUS ROUND recall.  Just act really matter of fact and get out of there and return to your previous training point.
• Next, you'll take him through the gate.  Start on a long line.  Open the gate and let him wander through.  Call him and reinforce him for any motion in your direction.  Walk backwards to make it really clear what you want (this is important when we graduate to off leash through the gate).
• Next go through the gate and towards the horses.  Go through all the steps over again.  By this time, you should have a relaxed, matter of fact recall, especially on the long line.  I'm working towards a default offleash heel during this exercise, so I'm constantly moving.  I think this keeps it interesting for the dog.  Go from a short length of line to the full fifty feet.  Recall from all over the pasture - standing near the animals, with the dog near the animals and you near the gate or heading in that direction, with you out of sight if possible with the line.
• Add an intermediate step where the dog drags the line and go through all the above steps again.
• Finally, go through the gate with the dog off leash entirely.  The first time, walk through the gate together then without saying anything, turn around and walk back out, then recall the dog.  If you've done your homework, the dog will come happily.  If there's any hesitation at all, got back to working the gate on line.
• Go back and forth in and out of the pasture with the gate open (assuming it's safe for the horses) - first walk in and walk right back out, then go a little further in and watch for that little twitch that says, "I'm thinking about some equine action" and try the recall.  If you don't get an immediate and relaxed response, try about a million repetitions of the previous step.  Your dog should b a) bored to tears with the idea of going in that pasture and b) really interested in you.
• The rest is just increasing the distance you allow the dog to get, from you, in that pasture.  It should be old hat for both of you by now (you meaning the owner).

The trickiest parts are the first time the dog has free access to the horses, and the first time you go through the gate.  It's all about trust there.  What helps me is to think, "What's the worst that could happen?"  If that consequence is unacceptable, then don't train in such a way that that is a possibility.  For instance, if the horse is dangerous, don't train around the horse and make sure the dog has no access to the horse by tightening the fence.  But the recall still is needed for emergencies, so you train the best you can - even going back and forth through the gate and training on leash in the pasture is better than nothing.

If the dog will recall out of the pasture, after you've walked through the gate, leaving him there, you can call your plan a success.  Good luck!