A lateral walk is not a clear four-beat walk. It is a pace in which the legs move on the same side in unison. Unfortunately, it is sometimes the best walks that are most in jeopardy. For example, a horse with a very scopey natural walk may run the risk of developing a lateral or pace-like tendency, especially as you start to collect it. So, many people have been told to leave the walk alone for fear of making it worse. But there comes a time when the walk needs training.
If you haven't been methodical about making sure that the connection and ahead-of-the-leg requirements, which were trained in the trot and canter, also apply to the walk, then that is the first issue that will need to be addressed.
You need a connection in the walk that is even and accepted. The horse needs to go to either rein from either inside leg, and you should have sufficient neck control, including being able to walk the horse on the bit with the poll at the highest point, in left or right flexion. If that sounds hard then you are starting to get the point. Establishing a good connection is hard, and it needs to be developed correctly and methodically to maintain a feel that is comfortable for both horse and rider. If the connection is not honest and accepted in the walk, it probably needs improving in all three gaits. The walk is just the hardest place in which to hide any connection or rhythm issues.
When we start to work on the connection in the walk, a common problem is the horse gets tight in his topline and back. For example, when you pick up the reins, the horse starts to back off from the connection or breaks into a jog. This often causes the rider to hold on to the connection in a restraining way, which prompts the horse to back off even more. Add any unevenness of rein or leg pressure and now your horse is not only behind the influence of leg and rein but he is also crooked. These are the things that can lead your horse toward his first lateral steps.
Here's another scenario. The horse may flatten his back, push his withers down (which will back his shoulders into you) and refuse to accept or go to the connection. This causes the hind feet to be slow in leaving the ground, another cause of the lateral walk. In short, most lateral walks stem from improper connection coupled with a horse who is either crooked or behind the leg. This often happens with horses who have a loose walk with lots of scope. Horses with short, tight backs, may also be predisposed to some lateral tendencies.
Riders often pay a great deal of attention to these factors in the trot and the canter and then let them slide away in the walk--especially when the walk starts to look questionable. The rules of connection, flexion, throughness and ahead-of-the-leg don't stop when riding the walk. They are, in fact, most vital, and it is my belief that many piaffe irregularities can be traced back to connection problems that show up in the walk.
But what happens when you address these connecting issues and the horse starts to get worse instead of immediately better? When things do not immediately proceed as anticipated, some riders will baby the horse in the connection or become vague about the expected answers. This leads the horse deeper into his problems because he needs step-by-step guidance from the rider with expectations that are continually defined and refined.
For example, you pick up the reins and your horse backs off or takes a couple of shuffled steps. You worry that you are making things worse and give up your connection and your leg aids. Your horse, in turn, learns that he does not have to step through to the rein. Now what? It is up to you to have a clear view of the desired outcome in your mind and then formulate a plan that will lead you to that place. All of the old masters suggest that you ride shoulder-in to correct walk problems. Why? Because the horse has a connection in shoulder-in, he has flexion, and he has to be in front of the leg. These are the qualities that you need to ride the walk well. The three qualities of throughness: connection, flexion and in-front-of-the-leg. If you neglect these issues in the walk, they will eventually catch up with you.