because it is not a pack of horses.
A Herd of Horses or a Team.
I believe that since we are talking about 1 group (singular) the correct way to say this would be 'A group of horses is called a team'.
2-4 horses
Agricultural horses can be called a team, Race horses can also be called a field Regular horses are more commonly called a herd
It is called drill team.
You could say a pair of horses, or a team of horses (if they are pulling something together).
Some collective nouns used for horses area stable of horsesa team of horsesa drove of horsesa herd of horsesa troop of horsesa sting of horsesa harras of horses
The Denver "Broncos"
It can be called a 'team' or a '4 horse hitch'.
a herd Horses (General) Team, Harras, Stable, Troop, Stud (a group belonging to one owner) Horses (Colts) Rag, Rake Horses (Ponies) String Horses (Wild) Herd
they are called a team;-]
Indianapolis Colts