While Acoustic may not be a true genre, it's definitely a popular style in the brony music community. Stripped down acoustic guitar or piano; it's minimal, often gentle and mellow, and it's full of emotion (and every once in a while it's actually produced with a DAW and VSTs despite the sound). Whether it's classified as folk, singer / songwriter, or just a simple instrumental, you'll find quite a bit of great acoustic pieces in the brony community.
Check out these artists who specialize in this style of music.
Feel free to recommend some artists not included in here (these artists specialize in it) or include up to 10 of your favorite acoustic tracks in the comments section (for an eventual best of countdown).