Randomly picking on Pepper, because picking on my friends is a passtime of mine!
Pepper has a weakness for the big-boys: stallions, cave-wolves, etc. And of course the question comes up - where does she put it all? Of course the old AD&D "bag of holding" came to mind, and so "mouse of holding" just seems to make sense. I opted not to go where Zaush did with the "Otter Space" gag, and just leave it up to folks' imagination what goes on when she's trying to live up to her role.