cos Goats and some frogs have horizontal pupils. Naruto's sage mode is modelled after a frog/toad.
Horses have the oval pupils as well, these eyes help them see better in light and dark since like all pupils, light enters through the pupils and is reflected back from the optical nerve which translates the shape of an objects. It also involves with being a predator or prey animal. Horses like goats are prey animals and have a 350° monocular vision and a narrow binocular vision (the eyes are on the side instead of in front like humans). By having such a long range of site helps them see incoming predators and them high tail it out.
I'm taking an equine class for veterinary major and we just learned about horse's eyes
Why are all goat's eyes look like eyes from Naruto in sage mode