A quick dirty edit of this awesome piece based on a comment…
 
After a long night, Luna wanted nothing else but to have a nice hot bath to relieve some stress and be alone. After sitting in the tub for around an hour, the Princess began to feel the once perfect water begin to grow cold, so she gingerly unplugged the drain…only the water didn’t drain out of the tub…instead she felt a tingling sensation between her legs as the lavender water began to enter her body. She moaned as the water pleased her royal pussy as the water level began to fall in the tub, and her body began to bloat. Soon, Luna was completely and embarrassingly stuck as the last of the bathwater swirled up into her ass, leaving an empty tub and an overfilled mare.  
“SISTER?? Did thou cast a spell on my bathtub??” Luna shouted  
“…Maybe…” came a sarcastic reply from an adjacent room.  
“VERY funny sister, now come and get me out!!” she demanded.  
“…sigh, fine, I guess I’ll help you…”