It's called "Secret Histories", and you can find it here.
You'll notice that I've got a custom genealogical diagram as the cover art. As I was drawing that up, I realized that I never came up with a cutie mark for Butterbold.
Well, we can't have that! So I inserted the following into Chapter 23 of "The Best of All Possible Worlds", when Blue Belle first meets her:
She arrived at the Council chamber a full half-hour early. Standing apart from the members of the Council was a tall bright yellow pegasus with an orange mane and the cutie mark of...the most adorable rattlesnake that the unicorn had ever seen. Blue Belle supposed that she was the witness that had been requested for the refugee issue.
Here it is in her genealogical symbol: