Talk:List of ponies/Mentioned ponies/@comment-10963749-20130617203034/@comment-4531340-20130617203726

Derpy is used in the show explicitly to refer to the wall-eyed, gray Pegasus mare, so that name takes precedence over "Ditzy Doo" (although that name was used in WWU, ofc, and at one point, The Last Roundup's script called for Ditzy Doo, though it was changed to Derpy by the time the episode was sent to be broadcast).

Also, that second sentence is fanon; there's been nothing in the show to confirm or refute that.