Suppose you have a horse that usually gaits at liberty, but has lost significant weight (from being overweight), is conditioned up, no tight noseband, and has a new treeless saddle.

All of a sudden, the horse doesn't "tolt" anymore, and prefers trot.

Odd, since the horse hadn't been seen to trot at liberty, up to this point, by the current owner.

Let's take a look at a few things and some biomechanics.

In the big picture, the trot is a common gait in all domestic quadrupeds.

Even people are diagonally gaited! When we walk, we advance a left foot / right arm; right foot / left arm. Same as with crawling!

Though quadrupeds have just a few defined gaits, an individual animal may exhibit great variation in gait. Some of the variation is due to species or breed conformation differences, some is due to training, experience, or health status, some is due to terrain, emotional status, degree of exhaustion, etc. Ultimately, an animal can be expected to choose a gait variation that is the most convenient under existing circumstances.

Locomotion and diet are linked in evolution. Carnivores require multipurpose limbs (used for both running & manipulation); whereas, herbivore limbs, devoted entirely to running, can be more specialized for locomotion.

Herbivores with roughage diets and bulky abdominal visceral have less flexible trunks and rely more on limb elongation.

Carnivores are fast because they have flexible trunks, which is possible because they have a small-volume meat diet (which is why they need multipurpose limbs in the first place).

The trot is the natural foraging gait of most wild animals.

The trot is well-suited for rough, irregular ground and for traveling long distances at a fair rate of speed. Work is spread evenly over all four limbs, and diagonal support makes it easy to maintain equilibrium.

In lateral gaits, the trunk undergoes lateral oscillation as right and left limbs impact the ground beside a center line. Equilibrium is OK with the pace, although the gait is not as stable as the trot. It seems to require less muscular exertion than the trot (less vertical oscillation).

The pace is uncommon among domestic quadrupeds. Dogs and other domestic animals generally pace because of fatigue or a physical weakness. Dogs that are obese or out of condition often pace rather than trot. Also, dogs that have problems with interference, may find that it is easier to pace.

Puppies and foals often pace until their muscles are better developed, at which point they switch to the trot.

The pace is a natural rapid gait for the camel, giraffe, elephant and bear. These animals have significantly different boney spinal structure than horses.