That is your horse's "Corpora Nigra" and very VERY normal. They are more noticeable on certain horses but every horse has them. They help shade the iris (pupil?) from sunlight, kind of like sunglasses. In some rare cases they can enlarge and affect your horse's eyesight but the size on your horse is just fine. :)
But yes, 100% completely normal, unless it grows too large then laser surgery needs to take place to cut back the access "cauliflower". But I doubt that won't happen.