This never really crossed my mind why animals have different pupils but a conversation around the fire yesterday opened my eyes to why they all have different pupils.
Eyes come with all different shapes and sizes
For Instance some animals like the Fox , House cat and Snakes have vertical slit eyes
Animals Like Goats and horses with Horizontal pupils
Or Humans, Tigers and Lions with circular pupils
The Theory about shapes is that vertical pupils allow for animals to increase and decrease their size of their pupil by a factor or 135 ( Unlike humans who can only change it by a factor of 15)
Below is an example of a domestic cat and its pupil change size
Vertical Eyelids are therefor very common with animals that are active during the night and day. In the image above the cat on the right has its pupils fully opened allowing a lot of light in helping it to see at night.
But this cant be the only reason for pupils to have these shapes.
A new study suggest this, For creatures with vertical eyes they have found that they are usually on ambush predators and they have their eyes on the front of their head. For example Cats and Foxes fit into this category perfectly as they are both ambush predators. The big proof is when you look at wolves and foxes, they are closely related but their eyes differ. Well Wolves aren't ambush predators, they hunt in packs and chase down their prey.
In a computer simulated test they found that vertical shaped pupils allow animals to judge distance without having to move their heads and give away their position to their prey.
So What about horizontal eyes
Well these are very likely to be on prey animals, goats, sheep ect.
They found that their eyes are usually further apart on their heads and while grazing they need to be able to watch out for attackers. The horizontal shaped pupils allow for these animals to have a wider view and take in more light giving them an enhanced view of horizontal planes giving them an advantage when trying to run away.
Vertical Eyes are usually only found on animals that are closer to the ground as they benefit most from it.
Circular pupils are usually found on all the exceptions from the above
I hope you guys also learnt something today from my TIL, if you did please follow me 
@warrensteem
Sources :
http://theconversation.com/revealed-why-animals-pupils-come-in-different-shapes-and-sizes-45796