Any toys hung this way stimulate the horse to swing them and generally knock them around. They can also be placed on the ground. There are also durable inflated balls that have a handle that assists the horse in picking them up with their mouth. They will shake them around, run with them, nose them and even kick them across the ground. You can often see them tossing them up in the air and then trying it again.
Some horses like to untie things.
You might tie multiple knots in a piece of cotton or natural fiber ropes and attach them around the barn and stall area for them to untie. This allows the horse to engage in the activity and keeps them out of trouble. Another type of enrichment is through physical activity. Horses need to get out into different environments and to run, roll, and frolic. This helps them to be better working animals.