[Gallagher]

http://youtu.be/ZaUbi-7uBDg

Derpy is a pony that originally just filled in a spot in the background of generic crowd scenes. In one shot, however, she was shown with crossed eyes, something that was just a little fun the animators had since she wasn't supposed to be shown fully anyway. A change in the scene let it be seen, and viewers didn't fail to notice. They stuck onto her and made her an entire character in her own right, making her popular enough for the makers of the show to actually add her as a reoccurring character for season two and onwards.

Yep.

Ponies.