My dog has developed a fear of horses?

If you got kicked in the chest by an angry horse, how willing would you be to go back?? I would get him out of the car, and let him do his thing. He will eventually wonder his way around, and become comfortable again. Entice him with some snacks, and baby him a bit. He will be fine, give him some time, and don't force him. Even if you need to leave the car door open, and walk away. He will explore.