There are some really good websites about a horse's field of vision. I think that the vision of a horse would make a great science project. Especially since what a horse sees isn't transferred to the other side of it's body.
The charts could be very colorful, You can put pictures of up close horse eyes.
I'm trying to picture the finished product and what not. Are you going to use the tri-folded cardboard thingys for your project? I think that you could find some really neat info on this and it would be fairly simple yet really informative since a lot of people don't understand how a horse sees things.