1.) Rainbow Dash wets herself yet again (after repeated wettings over the last two days inside the snowman, likely due to drinking lots of energy drinks beforehand to stay awake) after her sneak attack fails.
2.) Rainbow Dash managed to hold it in the entire two days as a snowman, but after Rarity blocks her sneak attack, she indignantly wets herself, frustrated that it was all for nothing.
Couldn’t resist one last one because I love to headcanon that she did this, as she REALISTICALLY would in this situation which she willingly put herself in and chose to remain in no less.
At the same time, to anyone who dislikes this kind of thing, do NOT let my inclination ruin the scene or the episode for you, because the situation is so absurd in every way that you have to call Rule of Funny for all aspects of it. First, Rainbow should also be hungry and tired from lack of food and sleep yet she’s energetic, and she’s not shivering either despite being exposed to the cold for so long. Also, Rainbow Dash certainly isn’t the type to stay still in one place for even an hour and you’d think she’d want to get in on the action for the entirety of the snowball fight, especially with her speed. Not to mention if she went to such lengths for a sneak attack, why would she loudly announce it, thus giving Rarity plenty of time to counter it? And if the snowball fight was planned this long in advance, how come Pinkie either forgot or was unaware of it? All Rule of Funny so she probably canonically stayed dry as well, especially combined with the same fiction physics like when the Main 6 were locked up for the same amount of time by Starlight without needing to pee even once (given lack of any puddles or desperation), despite Pinkie having needed to pee really badly three seasons ago.
So, if you saw it as simply an amusing joke, go on seeing it as such, that’s probably what it was meant to be first. (Won’t stop my headcanon but that’s just me) Besides, they did deliberately give us heavier implications of Fleur wetting herself in Five Lines You Need To Stand In - and even then the outcome is ambiguous so she COULD have made it.
Edited