I think my all-time favorite piece of original pony fan music is Discord, by Eurobeat Brony. You can listen to the original version in the video, but this post is actually about one of the many awesome covers of this awesome song.
Thunder Dash, who is crazy-talented at writing piano arrangements, has done what he does best, this time to the tune of Discord. You can listen to Thunder Dash's piano version here, and download the sheet music for your own Discord-related musical needs.
Thanks so much, Thunder Dash!