I have a different take on this. I have seen so many of these horses that I have to look at what all of them have in common. It is so common, that I have put a lot of thought into it.
I have been working occasionally with a local family that has grand kids in the 4-H club that meets to ride at our place. Three of their four horses have a huge aversion to loping. Their old 19 year old gelding they raised and trained with our help when their kids were in 4-H is the only one that rides right. He took their kids to the state 4-H show for several years and placed regularly in Pleasure, Trail, Western Riding, Horsemanship and Showmanship at the State Show. They won many 'high point' trophies with him and a younger mare by our stallion out of his mother. [This mare died in a pasture accident several years ago.] She had also won quite a bit including a 3rd or 4th at State in Western Riding. She turned out to be a fantastic 'lead changer'. So, they KNOW how a horse should ride.
After that, the kids aged out of 4-H and had kids of their own. In the mean time, they bought a nice mare at the local sale (a broodmare that was headed to the killers -- gentle but un-ridden), one they raised by one of our stallions and one I gave them as a yearling. They did all of the training on the last three at home without any help. Since the kids had trained the others under supervision, they thought they could train these new ones without any help. WRONG!
They have trail ridden these three horses for several years. I offered a few tips on the big trail rides (like the St. Jude ride) and invited them to bring them over when they explained what these horse would NOT do. They would not lope. They could not be guided going any faster than a walk.
So, they started bringing them over with their grand kids that are too young for 4-H. These horses all rode just like this horse . They would trot 90 miles an hour, lope a couple of strides, break back to a trot and refuse to lope again. They were stiff and miserably uncomfortable. At the walk and trot, they dropped their heads and completely relaxed. They had literally talked themselves into thinking that the lope was the most miserable thing in the world to do. They had all developed a terrible aversion to loping.
Horses can easily develop an aversion to anything they do not understand and are allowed to determine for themselves whether it is good or bad. They, by nature, are not comfortable doing things they have not experienced previously. What they accept on the ground does not always influence a lot of what they accept under saddle.
They are very much 'creatures of habit'. The very first time you ask a horse to lope under saddle is VERY important, as is the first time you ask a horse to do anything new. This is why we always say "Do not ask a horse to do anything he is not ready and able to do." Then, once you ask, you must be able to make the horse do it if he decides not to. If he is really 'ready and able' (well prepared) for the next step, there is seldom any need to 'make' the horse do it. This judgement comes from experience and skill.
Back to the three horses that came here unwilling to lope: What had happened to them and about every other horse we have seen with this problem, is that the horse was not moving forward willingly enough under saddle when it was first asked to lope. On top of that, the rider did not have the skill or resolve to make it happen once they asked. With all of this uncertainty, the horse decided not to lope. Once the horse decided not to lope, the tone was set. Any attempts made after that were met with more and more resistance. This is where the bucking comes in. I have even seen horses try to bail out of a round pen trying to avoid loping. I have seen many horses get so determined to NOT lope, that they almost made it a 'do or die' situation. Oddly enough, some of these horse loped very nicely on a longe line without a rider. Bucking is a frequent type of resistance.
Here is what we did: We put them in the 60 foot round pen to assess their gaits and their determination to NOT lope. If they loped nicely from the ground, we tried to work them with a rider. We just asked the rider to stay passive and NOT do anything. Just go along for the ride. Sometimes this works, but usually not.
It did not work with any of these three horses. I just had one of their sons literally force the horse to lope in the round pen. He spanked the horse's butt until each horse finally broke into a lope. I had instructed him to get one lap around the pen and then to pull the horse up with the inside rein. It is very important that the rider determined when the horse drops out of the lope. The second time took a little less encouragement and the horse went a little farther before it was pulled up. Then, we went to the 150 foot round pen. After it loped a little more than a full lap, it was pulled up. It did one lap each direction and I had them quit while they were ahead. They have only been coming over once every month or two, but each horse is getting a lot better. They can now lope several laps around the big pen and can be guided across it without stopping. They have all loped out in the pasture and will all lope ahead of other riders out on the trail. They will probably not ever be as comfortable loping and would be a lot more difficult to achieve good collection and self carriage with than horses that had been ridden correctly from the beginning.
When I go back and look at all of the horses I have encountered that had an aversion to loping, almost all of them had similar back-stories. They had been asked to lope and then allowed to NOT lope because they decided not to lope.
The exception would be the horse that had a injury or some physical problem. Then, the error in judgement was made because they were not 'ready and able' to lope when they were asked.
The biggest single problem is not having a horse forward enough to WANT to lope. When we start colts, we frequently lope them on their first ride. Then, it is usually a matter of getting out of their way and 'letting' them lope. It is ALWAYS very important to never let the horse decide when to stop. The rider has to call all of the shots or the horse is always resisting and trying to take charge.
Really, it does not matter if a person asks for a lope on the first ride or the 20th ride. What matters is that the rider and the horse is ready to lope and the rider determines that the horse WILL lope and the rider determines when the horse is going to stop or drop back to a trot. That is what really counts.