When at full gallop, a horse’s rear hooves make contact with the ground almost at the same time, but not quite. While watching the Belmont in slow motion, all the horses that I could see clearly always touched their left hoof first. Is this true of all horses, or are there also “right-hooved” horses?
In the canter and the gallop, the horse uses a “leading leg” This is the leg that carries the full weight of the body when all the other legs are in the air (just before the moment of full suspension - no feet on the ground). The leading leg is normally the inside front leg on a turn, which would be the left leg on a left turn. As the horse’s body is bending left, i is easier to bend toward the supporting leg than away from it, as would be the case if you had the right lead on a left turn- aka “counter-canter.”
(see stop action photos of a galloping horse)
Cantering/galloping on the opposite leg, while possible, is more difficult. I believe racehorses are trained to swap leads on the straightaway, to avoid overtiring themselves on the left. (leading leg doesn;t matter on a straightaway).
Horses ARE born right or left-sided, just like people. It is easier for them to pick up the lead on one side or the other, depending on their sided-ness, but it is not hard to train the other lead, for the most part.
A Mr. DE Williams of from the California State University-San Marcos headed a study into the stride pattern of racehorses. To cut a long story short, he found that about 90 percent of racehorses preferred the right lead.
Link.
The differences in stride patterns in right and left lead are illustrated here.
As Hello points out, racehorses are trained to swap leads. This is often necessary in changing tack in order to pass other horses in the race.
More to the point, it is tiring to run on the same lead all the time. They run on the one lead (Right IIRC) in the straight, switch for the turn, then switch again for the stretch run.
Someone doing a Borel-style ride jinking to find holes, they might change leads more often.
And a horse trained to an advanced level in Dressage can change leads on every stride. Here is a horse doing “one tempes” - at about 0:50 into the video.
There are plenty of tracks over here with straight races up to 1 mile i.e. no bends. I’m just wondering whether, and if so how many times, a horse would be asked to change its stride pattern in such races.
Also, I was under the impression it was the jockey who ‘encouraged’ the animal to change its lead. Is this correct? (I do know that a horse will change its legs voluntarily if, for example, it is racing on a very firm turf surface to which it is unsuited).
Lead changes are a pretty big deal at almost any level of horsemanship, especially athletic horsemanship, stuff like jumping, barrel racing, cross-country, steeple chasing and flat races, calf roping, dressage, formation drills. It is done with subtle shifts of the riders weight and after some training leg and reign cues. If done wrong or not done at all you end up with a horse that is “wrong-footed.” That can lead to stuff like hitting jumps, falling into ditches and general unpleasantness. As the old army song said of one of the staff horses at Ft. Riley: “a splendid four gaiter they called her, walk, trot, stumble and fall.”
You get the same thing with human hurtle racers when they get their steps between the jumps wrong and have to take a stutter step – they were wrong-footed.
I thought there was a difference between canter and gallop, am I misremembering ?
Walk - four beats : front leg, opposite rear foot, front leg, opposite rear foot.
Trot - two beats : diagonal pairs.
Canter - three beats : leading front leg, diagonal pair, remaining rear leg.
Gallop - as walk but faster, all four hooves off the ground at one point.
Anyway I guess this means that any one horse will have a natural leading leg in all gaits ? I also remember changing the leading leg while cantering but never at the gallop … is it possible to do so or is the horse travelling too fast ?
The horse will have all four legs off the ground during the canter as well, but the overall feel of the gait is three beats. The gallop doesn’t have the diagonal pair on the ground, so the overall feel of the gait is four beats.
Some horses never get really good at changing leads at the canter or gallop. This may reflect a lack of training or simply a lack of ability. There’s a reason that there are many more horses making muffins in a pasture than making videos like CannyDan’s example (and I say this as an horse owner and lifelong horse lover).
Right, I only meant to show that both the canter and the gallop require a leading leg, not to say they were the same gait. I used to tell my riding students who were musically inclined that technically the canter has 3 beats and a rest. The rest being the moment of suspension. 
The gallop has 4 true beats and a rest also, but has a very notable “jerky” front and back motion, as compared to the “circular” feeling of the canter. A gallop is not just a fast canter.
Then you have the great ones that come equipped with “auto changes.” My old mare never took a wrong lead in all her years, and it was rarely due to any help from me. One time at a show I was annoyed when she broke from the canter… but you could see on the video that she was correcting her lead, which I had not noticed was incorrect for the turn. Smarter than me, it would seem! (lead changes may be “simple” - the horse briefly trots then picks up the correct lead - or “flying” - the horse swaps mid stride. The flying change is possible at the gallop - obviously the trot is contraindicated in a horserace!)
Horse do favor a side, in all things, for example, they would find it easier to bend their body in one direction than the other.
Just wanted to add that most horses prefer the left lead, probably because they’re subtly trained to be more supple that direction from always being led on their near (left) side.
Only as often as needed to keep the horse from getting tired on the lead side. Possibly not at all in short straight races (Quarter horse races, for example).
During a lead change, even a flying one, a horse effectively slows down (travels less distance as the stride changes), so you do not want to do this more often than necessary in a race.