First of all, the very funny new My Little Pony cartoon series, Friendship Is Magic, was renewed for a second season recently (I'll be writing a full review soon, but trust me, it's very cartoony and very very funny). Lauren Faust is still the exec producer. This is important, I'll get to it in a minute.
As I've been catching up on the My Little Pony cartoons, I noticed something. I don't know which episode it was, but I did notice this character pictured here on the right. She turns up once in a while, and I find myself sometimes scanning the crowd scenes looking for her.
So who is she? A search on the Internets reveals she is not an actual My Little Pony. There are sites that list every single pony, names, what they look like, etc., from the time the toy line was created in 1985 to the present. There is nothing on this one. She does not have an actual name, but fans have taken to calling her "Derpy" for obvious reasons. There is some speculation that she is a character referred to but not seen in another episode, where someone asks about a pony named "Ditsy-Doo". We now have proof that this isn't the same character, I'll get to it in a minute.
Derpy appears sometimes with the derp eyes and sometimes with normal eyes. It seems that she isn't stupid, she just has that look. From what I've found, her first appearance was in a party scene. Because the episode is done with Flash animation, things can be moved around at whim to make the scene look better. Derpy was originally in the background, and the eyes were an animator's mistake. But Derpy was obscured by another character and her eyes weren't visible. Nothing to fix, right? But the scene was sent back to be redone and the characters were spaced out a little. The one obscuring Derpy got moved, revealing her expression.
Well, the animators got a kick out of it. Lauren Faust said in an interview that they took to digitally inserting Derpy if they could, at least when she wasn't being featured prominently in a scene.
Now, here's where the loose threads at the start of the article come together.
Faust was being interviewed about Season 2 of FiM. When asked what changes were coming, she said none -- the show's a hit with its current look and vibe, and they are leaving it that way. Faust did mention that there will be one change, though. The fan reaction to Derpy has been big. She says that, instead of sneaking her into the backgrounds, they are going to feature Derpy more prominently.
She specifically referred to her as "Derpy". Hence, it's not Ditsy-Doo.
An animator's mistake has become canon.
Like I said, you just never know.