Like most animals, horses have an acute sense of smell that they regularly employ to provide them with information on what is going on around them. Horses use their sense of smell in a number of different and important ways.
Nature equipped horses with a strong olfactory sense that can tell the animal whether a predator is near. All it takes is a strong upwind breeze to bring a dangerous scent to the attention of a wild herd. After getting a whiff of the predator, the herd literally high-tails it (their tails stick way up in the air as they flee) out of there in a flash.
Horses also use smell as part of their complicated social structure. Horses typically greet each other nose to nose, each taking in the odor of the other. Horses also come to recognize each other by scent as well as by sight. Mares and foals quickly memorize each other’s scents and use this information to help locate each other in a crowd of horses.
So how should I approach a horse?
Most horses also greet humans in the same way. When you introduce yourself to a horse for the first time, notice how the horse reaches out his muzzle to sniff you. The most polite way to approach a horse is with the back of your hand extended so the horse may take in your personal scent. Letting a horse breathe in your scent tells the animal that you are a fellow herdmate (not a predator), and usually makes the horse more agreeable to being handled.