If you read yesterday's post you'll notice that the racehorse with blinkers also had what looked like tape on his muzzle.
Or maybe it's a band aid?
It's a nasal strip. Human athletes use nasal strips to hold the nasal valve open and prevent it from collapsing under pressure. You see them mostly on football players because they're wearing mouthguards and can't breathe through their mouth - which means they need that extra bit of airflow.
Horses also cannot breathe through their mouths. However, the nasal strip is not used primarily to enhance performance (although some racing jurisdictions ban it as a performance enhancer) but to help prevent bleeding in the lungs. Equine nasal strips were first seen in the 1999 Breeders' Cup. They are also sometimes used in the cross country phase of a three day event.