A group of horses is called a "team" or a "harras." If all the horses in a group are colts, "rag" can be used, and a group of ponies is called a "string."
Countless terms are used to refer to a group of horses, and many simply call them a "group." "Team" was especially popular when horses were used together to pull items, and the term maintains its popularity today. Calling a group of horses a "harras" is uncommon, but it is still heard on ranches throughout the English-speaking world. Less commonly, a group of horses owned by a single person may be called a "stud."