* They both exist in the same environment: Horses and flies are often found in the same places, especially in fields and pastures.
* They are both prey for other animals: Both horses and flies can be prey for different types of animals. For example, a horse can be prey to a lion or a wolf, while a fly can be prey to a spider or a bird.
* They both have a relationship with each other: Flies often annoy horses, and horses try to swat them away!
Let me know if you had a specific answer in mind, and I can try to come up with something more specific!