I've always wondered what is the sticker that we often see on some horse muzzles in eventing?

Oh. I think I know what you are talking about. Those are breathing strips. The strip opens up the airway during competitions so they perform better.
They are flair strips, it helps the horses breathe better and get more oxygen through the nasal passages because usually during heavy exercise the horses nasal passage can collapse and it's harder for the horses to get enough oxygen.
Join the fun and sign up to connect with our 200,000 members!