I don't know which forum this fits best, I'm putting it here because from what I've seen it appears to be the best fit. Anyway, one thing I find interesting about brony music is not just that there is some very good songs about horses, but in some cases the brony music scene ended up producing unique music, and it's musically unique BECAUSE it came from the brony music scene, it's not just that a musician with a unique style coincidentally happened to make a song about horses. The reason for this phenomenon is that the brony music scene is remarkably well connected, but also extremely diverse, leading to collaborations and fusions that otherwise would not happen. For example, I've noticed that brony industrial metal in particular tends to stand out from other industrial metal, and that's because it's roots are not in other industrial metal, but from metal musicians who were inspired by electronic artists within the brony community. The best example of such is Tarby, and here is one of the songs this influence can be heard best, particularly during the bridge:
The thing is Tarby is not alone. Another good example is DivinumX's pop-tinged extreme metal. I think HeyLasFas also fits in the same sub-scene, whose album Destiny being an absolute masterpiece. Then there are several collaborations that push boundaries in similar ways, such as Breaking Bounds by Princewhatever.
Reason I'm posting this is because I'm sure there is many more examples, but with my preference being towards metal my perspective is rather limited. I'm looking for more examples of musical innovation being driven by the brony music scene. I think the Brony Music Scene itself would be a good subject for a documentary, with the key point being that it's more than just songs about horses. Examples such as these provide a very convincing case that it is, and that's without going into all the friendships that formed.