|Hi, thanks for considering me when asking for a review!
I enjoyed the tone of your writing, and it's always nice to read a piece that is grammatically clean. You also have some really nice descriptions in your writing. It's too soon to see how Kacey's character develops, but I see quite a bit of opportunity there - You tell us what she's done without long descriptions about who she is, but your language is colorful enough to give the reader a picture.
However, I have two suggestions:
1. Avoid Information dumps
This is one of the more common writing mistakes and one I frequently fall prey to. What separates an information dump from a good narrative description is a little hard to define, especially for storied written in the third person omniscient perspective. However, it is best to avoid explanations of what you describe, such as "Buttercup had what was called Tobiano markings, which meant a large spot of white covering her left shoulder and ended right above her stomach." Your reader may not know what Tabiano markings are - I didn't. Unfortunately, you're probably going to have either leave the reader to look it up, describe the markings without giving the term, or give Kacey someone to talk to besides a horse that also needs the explanation.
I've also heard long narratives about things that a character already knows referred to as information dumps: most modern writing focuses on only painting a brief description of a scene, dribbling it out slowly as characters interact with it.
2. You never get a second chance at a first impression
Several years back, I began writing my first book. With perseverance, I completed the 130K word fantasy novel and placed it online, on a site that allowed me to see how many people read each chapter. My first chapter was mostly similar in tone to yours and served to introduce one of the main characters before everything started to go wrong. Fewer than 1 in 100 readers who were not friends or family read past that first chapter - the ones who did mostly had good things to say about the overall story, but almost all of them complained about the slow start. Attention spans for reading are shorter than they have ever been: if you don't hook the reader with the first chapter, you won't get another chance. And what mostly hooks people is a sense of conflict. It is common advice to begin a story "In Media Res" (In the Middle of Things). You do not have to do this, but you will want to give a reader a sense of tension or progression. By the end of the chapter, the reader should be unsatisfied: there should be something they want to find out. This chapter is idyllic: it's a description of a stable owner in bliss, enjoying her horses. It's pleasant, but a reader can exit the story here without feeling they're missing anything. You can't let them off that easily!
That said, I know some equine lover is going to going to pick this up, wanting to hear more about the horses, and be devastated when the conflict strikes such sympathetic characters. Good look telling your story and reaching your audience!