If you can, spend some time observing real horses and interacting with them. If you can't get to a place with live horses, watch videos with horses to get a sense of how they move and behave.
[header] How to write a fictional story about horses [title] Learn about horses. [step] If someone who knows a lot about horses reads your story, you want them to get the sense that you know what you're talking about. Read up on horses online and at your local library.