Although breeds can look wildly different, they all belong to a single
species: Equus caballus.
Over thousands of years, people created different breeds by mating
horses that have desirable traits. That's why the look of a breed is
often related to what it's used for. When these traits get passed down
over many generations, a new breed is produced.
For example,about 200 years ago, English horse breeders mated light,
swift Arabians with local riding horses. This created the Thoroughbred–a
lean, leggy, super-speedy breed used in horse racing.