There are a number of identifying white markings on a horse's face and they are so common they have their own names.
Horse markings - Wikipedia
static01.nyt.com/images/2014/05/24/sports/24socks/
Horse Face and Leg Markings - Horse Illustrated
Vitiligo in Horses: Causes, Progression and Diagnosis
Informational Archives - HorseLoverZ Blog
Horses Lovers: Horse Face Markings
Southern Phantom: The white-faced horse who's the talk of the
What are some of the most common facial markings for horses? - Quora
Color & Conformation – Pryor Mountain – Wild Mustang Center