Mendacity, by Dromicosuchus
Adventure

I think this story started out as a joke, like - "What does BonBon's voice keeping changing? Because she's a changling! lulx". That's the only way I can imagine it. However, it quickly evolved into a pretty epic adventure story that ties extremely well into the season 2 finale.

Bonbon is secretly a changling that lives outside of the acceptable customs of her species. She's secretly in love with her roommate, Lyra. One day, Chrysalis finds out where Bonbon is and kidnaps Lyra for her own nefarious purposes. It's up to Bonbon to find her friend before it's too late. Along the way she meets interesting characters and has to come to terms with her own biology.

One of the interesting things about this story is how deeply the author has thought about his mythology. I think there's more thought behind his changlings than any other fic I've read. He's created more species and a whole political system for them, which is fascinating.

If you like adventure stories, this is one of the best.