American Standardbred horses can be trotters or pacers and their name comes from the standard qualifying speed test they must pass to qualify for registration (a mile in under 3 minutes).
From One Source
All American Standardbreds can trace their lineage to the originator of the breed, an English Thoroughbred stallion named Messenger brought over in 1788. He was bred to mares from a variety of different bloodlines & breeds (usually gaited breeds) with the intention of creating a faster gaited animal for racing. Interestingly enough, ole Messenger never raced himself.
Trotting Vs. Pacing
Standardbreds can do it all, however there is a distinct difference between the two.
Trotting is a natural diagonal gait where legs on the opposite sides move simultaneously.
Pacing is a lateral gait where legs on the same side move simultaneously. Lateral gaits are ideal for harness racing and used by about 80% of harness racers.