As a follow-up on ‘pacing’ v. ‘ambling’ I thought I would turn to some authority. In this case, I will quote from “The Dog In Action” by McDowell Lyon, one of the first studies of the anatomy of a dog and how it affects their movement.
“In some fields, the pace is referred to as the amble.” (page 50). Thus, yes, the two are the same thing. However, note that Mr. Lyon discusses this gait as the pace throughout the book. He also correctly notes in more than one spot that dog breeders at shows will call the gait the pace, usually with derrogatory remarks. Thus, for dogs regarding conformation, the gait is usually termed a pace, not an amble. This also appears to be true for horses, where pacers are set aside from trotters in races (pacers go faster).
As for why dogs pace, we turn to page 51, where Mr. Lyon notes: “For one thing, evidence marks this [pacing] out as a fatigue gait, or a product of physical weakness. Foals and puppies often pace before they learn to trot, taking to the latter only after their muscles develop and become firm.” Later on the same page he notes: “Interference is perhaps another agent that makes a pacer out of a dog. Over angulated behind for conformation, and balance with the forehand, he finds it easier to exchange the diagonals of the trot with their constant clipping for the greater foot freedom of the laterals instead of continuing to run crab-wise.” So, in other words, dogs with poor conformation can end up hitting the forefoot with the hind foot of the same side when trotting, and have as the option crabbing (turning slightly to the side) or pacing, where the legs on the same side move forward at the same time.
“Beneath tall this there is perhaps one underlying factor which brings out this gait at such times; it is the animal’s reluctance to fight lateral displacement, an ever present force in all gaits which is recognized by many horsemen but by few dog breeders.” As he explains in a chapter on lateral displacement later in the book, this is the movement of the center of gravity on an arc that runs from one diagonal of support to the other diagonal of support as the dog shifts weight from the one diagonal to the other. So a dog, instead of fighting lateral displacement utilizes a gait that converts the displacement into a help, such as by pacing, where the shifting of the weight side to side can be used to help increase the movement.
To my knowledge, there is no breed of dog that in its standard prefers pacing to trotting. But if you see a dog pacing, it is doing so either out of fatigue, or out of poor conformation.