This was requested of me yesterday, so I hurried and drew one up.
Keep in mind that this is just a basic, straight-on eye, and not every horse you draw will have their eye at this angle, with this lighting, ect. I hope this will help anyone who struggles in this area - if you have any questions, please do ask them!
And, if you use this, please let me know - I just want to see your work.
This is a 4x6, drawn with an H and a 6B pencil. No particular reference was used.