Here’s a few links to a series of articles about getting horses right in fantasy & historical fiction. But they are somewhat oversimplifications: I also highly recommend the Wikipedia article on coat colour in horses, and following up on the specific articles on coat colours that interest you. Through this, I learned that Lightning, my protagonist’s horse in the Wyrd West Chronicles, is most properly described as a buttermilk buckskin (golden-cream coloured with black mane, tail, muzzle, ear-tips and other highlights. It also means that his eyes are a chocolate brown.)
Horse Misconceptions in Fantasy Writing
How to Describe Horses in Fiction
Horse Terminology: Gaits and Anatomy
Horse Terminology: Tack and Riding
Diane Morrison 