Of course, our horses don't possess the sniffing ability of our dogs, but their sense of smell is far greater than our pitiful ability to detect a few smells. Horses throw their heads back and inhale, opening their nostrils wide to bring in all sorts of good smelling info. It's hard for us to begin to imagine all the info our horses gather from one good sniff.
Let's start at the tip of their nose - the nostrils. Big, flexible nostrils that flare wide open allow your horse to pull in all of the scent-laden particles packed full of message and info. Plus, they can breathe info huge amounts of oxygen, allowing them to run, run and then keep running.
The damp mucus within their nose act like smell grabbing flypaper and capture scent particles. These scent particles are then moved deeper into the nose where tiny hairs slow the movement down, allowing your horse to identify and categorize the smells. FYI: Dogs are able to actually send smells to a special compartment for later identification (smell bookmarking).
Horses have two separate and distinct olfactory glands/bulbs that handle the actual identification of the scents.
Like most mammals horses also have a very unique part of their nose called the Jacobson's organ that is 100% dedicated to sniffing out pheromones. And guess what, people do not have them. Wouldn't that save a whole lot of time and swiping left or right if we could just SNIFF to see if someone was a good match or not?
While this part of the nose is tiny, it affects the horse's life in so many ways. This is their matchmaking, is she/he ready detector!
You've probably seen a horse, usually a stallion, sniff the air and curl his lip up in a horse laugh look. The is called "flehmen" which translates as "testing" in which a horse is sniffing to see off there is a receptive mare nearby.
By pulling his lip up, the horse is literally closing off his nose from bringing in any possibly conflicting scents until he has time to thoroughly evaluate the interesting smell he just whiffed. It may look like a horse laugh to us, but to your stallion it is serious business!