Derpy is best pony. No, really, she is.
For those to whom Derpy is not familiar, originally she was just another background pony to fill crowds with. In the first episode, however, she was given a really silly cross-eyed scrunch-nosed expression. Viewers noticed, and she was almost immediately given the name "Derpy Hooves". From there on fans started fleshing her out as a character: She's a lovably silly and kinda clumsy pegasus who works as a mailmare, loves muffins, Is friends (sometimes roommates) with an earth pony named Carrot Top, has a unicorn daughter named Dinky, and may or may not be involved with Doctor Whooves.
Eventually, some of these elements were incorporated into the show itself. We see her working for a delivery service in one episode, she has a muffin-shaped saddlebag clip in another, and in one episode she even spoke and was specifically addressed as Derpy by Rainbow Dash. She's also excellently clumsy in almost all of her appearances.
She's not just any pony. She's OUR pony! ^_^