Here are some options you could use, depending on the context:
* A herd of horses: This is the most common and general term.
* A string of horses: This is often used for horses kept for riding or racing.
* A band of horses: This term is used for horses living in the wild.
* A troop of horses: This term is less common but can be used for a group of horses, especially in a military context.
You could also be creative and use a more descriptive term based on the specific situation, such as:
* A galloping group of horses
* A grazing herd of horses
* A playful bunch of horses
Ultimately, the best collective noun will depend on the specific situation and the desired effect.