Current poll results for potential additional cowpones in @ask-raricow:

1st Fluttershy
2nd Pinkie Pie
3rd Diamond Tiara
4th Sweetie Belle
5th Silver Spoon
6th Twilight Sparkle
7th Apple Bloom
8th Rainbow Dash/other (background characters/OCs/etc)
9th Big Mac
10th Scootaloo.

Be sure to vote at http://strawpoll.me/5721752 because the top 5 are almost guaranteed to become canon, and that may be extended.