A herd, a band, a harras, a stable, a stud, and a span, a string, a troop, a team, a field, and a remuda are all collective nouns for horses. A span is a group of two horses, while a string, a field, and a remuda are all terms used to refer to racehorses.
The etymology of the collective nouns for horses is rooted in the way horses have been used throughout history. A herd is a term used to refer to a group of horses that are kept together for grazing or other purposes. A band is a term used to refer to a group of horses that are used for work or transportation. A harras is a term used to refer to a group of horses that are used for military purposes. A stable is a term used to refer to a group of horses that are kept together for racing or other sporting purposes.