Rainbow Dash and Pinkie Pie really have to go to the bathroom. Rather than find a bathroom, they noisily soil their snow pants. They take pleasure in how it feels and find it kind of funny, where Twilight is weirded out. But she won’t let that get in the way of their friendship, even though she’ll have to put up with a very unpleasant smell for quite a while yet before Rainbow and Pinkie get home, shower, and change.
 
Of course Rainbow is wearing her skintight pants underneath. For Pinkie you can use your imagination.