Possible Clues:

• Turn signals
• They prevent a horse from seeing sideways
• In front of the car they don't give you much option to see where you're going
• They prevent a horse from having peripheral vision
• Joint during alcoholic drinks? When druggie ultimately emerges there's flashing lights!
• Eyes front with them
• Just see straight with them
• They inhibit vision, for person or horse
• Horse's eyepatches
• Leather flaps on a horse's bridle