Fluttershy: Oh dear! Rarity’s dangerous mission outfit is ruined! Guess we’d better go back!

Poor Fluttershy. Trixie made it so difficult for Snips and Snails to pull her carriage that she took a long time to get to Fluttershy and the beavers. As a result, Fluttershy peed in her costume. She just couldn’t hold it that long, plus she was pretty nervous about the whole thing.

She disposed of it in a river before finding Twilight, and later told Rarity that “the outfit was ruined.” Rarity didn’t press her for details and simply made her another one for Sparkle’s Seven.

(As of canon, given how rarely ponies need to pee based on The Cutie Map, I think it more likely there was just dirty water inside that tree, given the mud we see on her bunny ears and all the dirt on her, which does go with my platonic wam interest, but this is fun to fantasize about all the same.)

Sorry if this didn’t turn out adequately, I didn’t have a lot of good angles to work with. But I think it better as far as pony edits go to work with an outfit they’re already wearing in the show rather than drawing one myself. And in case you’re wondering why her entire butt is wet, it’s because she was laying on her back while peeing.