This mod SHOULD replace all horse riding music in the game with Old Town Road. Keep in mind that the horse riding music in the original game doesn't play ALL THE TIME when riding a horse, you have to be away from any places/scenarios that may play background music. The music also takes a few seconds to start playing. I'm not too sure, but I believe you have to reach a certain speed for the music to start. This is my first mod, so if it doesn't work I might not be able to fix it. However, if anyone wants the music to be louder/quieter I can do that easily.
Instructions: Place a copy of "BreathOfTheWild_Old Town Road Horse" from the .rar into your Cemu graphicPacks/downloadedGraphicPacks folder. That's pretty much it.
If following this doesn't work you can try to make your own graphics pack using the contents folder from "BreathOfTheWild_Old Town Road Horse." Honestly though, this probably won't fix anything.