They are called corpora nigra. All horses have a few of them in their eyes if you look closely. They are normally on the top of the pupil. They are thought to protect the interior of the eyes from bright sunlight. Some horses (very few) have "extra" in their eyes making them much more noticeable. One of my horses has extra in his eyes. I have had him since he was 4 and he is now 20, they have never caused him any problems. It looks like your guy has a bit more than mine does, but I don't think it should cause any problems for him.