I found this pretty cool guy that I really like, he knows how to ride, but his ground manners are awful. He did come out of an auction and I do believe he was a former polo horse (see my thread about growing out a mane and tail). But he seems confused by a direct rein, has no idea what “whoa” means (in the traditional sense?), doesn’t know “whoa” on the longe line, he just keeps on going and going at a trot. I tried the “whoa” command, tried to whoa him with the longe line, tried the “trill” but he blew me off completely. He doesn’t seem like he knows what to do with the bit, either. He worries it constantly.
So here is my question. Should I put him in a solid bit, perhaps a Kimberwick for some “whoa”? He isn’t necessarily running away, but if I ask for a trot, I can’t get him to come back to a walk without pulling on the reins more than I really like to. He is in a double-jointed snaffle with a lozenge now and he pays no mind to me asking him to whoa. My feeling is that is he was a former polo horse, he probably went in a curb or a gag and did mostly neck reining. I have no interest in riding Western, so I would like to teach him to direct rein, with as little force on his mouth as I can use. I am not going to put him in any Western types of headstalls, either. He needs to be able to ride in a bit.
Ideas, thoughts, suggestions?
Treat him as if he is totally green… because for your purpose, he is. You may think ‘he knows how to ride’ but he really only knows how to allow you to sit on his back with a saddle.
I would start him with a halter and lead rope, voice command ‘whoa’ then pull on the lead rope and when he stops…pat and treat. Repeat till he gets it, then the same with the lunge at the walk, then riding with a ground person and halter and lead over the bridle. If you can’t get good halt from a walk no reason to think it will work at trot or canter. Patience. Just like the mane will grow out, his training will fill in, be sure it is correct training.
I did work at a walk today thinking just that, no point on going into a trot if he doesn’t know how to transition the way I would typically ask, half halt and then reins. He is a safe guy so far, so I think I can use a bit and a bridle, I don’t think we need to back up to a halter. I think he knows a halt under saddle, for sure, I just don’t know how he was trained to do that. He has been ridden extensively, he is 17 years old or so.
OK - so polo ponies learn a very different set of rules than your basic hunter. Riding actually has nothing to do with it - obviously, the horse was ridden for years - but as a polo pony (neck reining, everything at breakneck speed, in a double bridle maybe, with lots of pulling on the mouth perhaps) Totally opposite from a hunter/english pleasure horse. Retraining a polo pony for a new job is not all that different from retraining OTTBs or some other wildly different discipline that you want to suit your needs.
You do need to start on the ground in a halter with walk, halt, back up - with increasingly less pressure on the lead rope to obtain whichever command. When that is mastered, move on to the lunge line and voice commands (walk, halt, trot, etc.). If that devolves into incessant trotting, go back to working in hand. I’m assuming this is a TB, so likely this horse is very smart and will catch on rather quickly. They love to please their humans! So we aren’t talking months of ground work.
I would also recommend getting the assistance of a trainer if you’ve never started a horse, training wise. And perhaps a book or two - look at the training books by Cherry Hill. She has several good ones. It requires a certain amount of patience and knowledge to get one going in the right direction. However, without the proper tools/instruction/knowledge both you and the horse I’m afraid, will end up frustrated. 
“What we have here is a failure to communicate.” Previous posters are correct in suggesting to go back to total green horse training. You speak English he speaks Zulu. Try to learn one or two words at a time. A PhD polo pony may surprise you how much he knows when you learn Zulu.
I have bought several adult horses that learned different methods of steering etc. I always start each horse from the beginning , as if it is an unstarted colt and teach it to walk, trot, canter, whoa etc using my cues. Horses try so hard to please us and he is only doing what he was taught by the last person. they don’t come out of the womb knowing exactly what we want from them. Please show patience when training him and when he even begins to do what you want , stop and reward him for a second then ask again and praise often, ignore the wrong response, reward the correct response and he will figure it out. Maybe take lessons so someone on the ground can help you also.
I’m going to tag onto the “start over” group, but will also add that there are a couple of other things you can try voice-wise first to check if there really is no pre-installed voice command for whoa.
Whistle softly a 3rd going down (sing doh re mi to get your notes and then whistle the “mi” and then immediately the “do” You might be pleasantly surprised. Or not.
Try a few more languages for whoa/stop. I trained a Dutch horse who was started in Holland. Eventually he learned English but he was always more reliable for voice commands given in Dutch. This is not normal, usually a switch of language is easy peasy, but if there is one that attached to his native tongue, maybe there’s another 
Good luck.
The point of going all the way back to the halter, is going through step by step to find out where the holes start. If you don’t find and ‘repair’ the disconnect below the break, it is always a weak point. So stat right at the beginning, if he is sound in his understanding then you can fast track until you find where the issues appear, then step back and work from just before that point.
When a horse is on the longe,and you ask for a walk, and get no response, make the circle smaller, and smaller each time repeating the word “Walk”, Immediately that he walks because the circle is too small, praise, treat, and then repeat, and repeat… Do not lose patience.
Under saddle he may well work off leg and seat alone. This is something I find many riders initially incapable of doing…
When you try using a direct rein, try turning your fingers up, and opening your arm out in an opening rein. Be soft!!!. Make it a leading rein.
You need to explain yourself to him. He doesn’t understand you.
You may need professional help.
Thanks, everyone for the suggestions. I noticed that his ground manners need a little work, since he does try to run me over frequently, but backing him up and circles do not seem to be getting the point across. I find it hard to believe he could have forgotten all of that? He does get pretty worked up about being away from him pasture mates right now. I am considering putting him on some supplements to help him out with the stress. He may need a little more time to do groundwork before I try riding him again.
Some of that may also be adjustment. I’m not sure how long ago you purchased him, but sometimes new people/new barn/new rules tend to cause a horse to be out of sorts. I use Warwick Schiller’s methods on a horse that wants to run me over (I despise having to wrestle a horse anywhere and do not wish to be dragged, pushed, or otherwise crowded by a horse who feels he has somewhere else to go) and highly recommend them.
I would 100% get the groundwork right before I ride this horse. I would also start from scratch again. If it’s in there, it will come out really rapidly, so starting slow will end up speeding you up at the end. In fact, if any horse of mine has ever taken significant time off I do the same thing. Some steps I might super speed through depending on the horse and their proclivities but it never hurts to reinforce basics.
Best of luck.
