Rainbow Dash really has to pee from drinking so much cider. Rather than hold it and try to make it to a bathroom, she just wets her pants right there. She seems to like it, and she can tell someone else does too. (Is it you? Fluttershy? Applejack maybe?)
 
I have a feeling a wetting-only one will be better received than the one that includes a visible stench from messing as well. Besides, I find ones that only show the wetness, along with ones that just show sound, are the best received of all my pee edits.