Rectangular pupils are not that uncommon. Many other herbivorous mammals have similar pupils, including sheep, deer, cattle, and many horses. You don't notice it as much with them, because they usually have a dark iris too. You can find rectangular pupils on other animals too, such as toads and octopi. Why a rectangular pupil? It gives a much wider range of clear vision, making them much harder to sneak up on. They can usually see you, even if they don't seem to be looking at you.