Derpy Hooves is background pony first seen in the first episode at the 17min-10sec mark. She is was in the crowd but she had eyes pointing at opposite directions , most likely an animation error, which some sharp-eyed 4channer spotted and posted a screenshot of it and then one user proclaimed, "The legend of Derpy Hooves is born!", and the rest is history. The show team noticed this phenomenon and decided to keep her eyes that way and showing her on the later episodes with her walled eyes as a small shout-out to the bronies. The show team went as far as to give her a small speaking role on the second season but unfortunately she was considered offensive towards the disabled and they had to alter her by correcting her eyes and changing her voice.
TL;DR: She is called "Derpy" because of her derped eyes.