Episode 8, no derpy eyes?
Here's some Derpy history.
In the beginning, Derpy was an animation error. Her derp'd eyes appeared during Episode 1. This error happened quite a few times later on because her character model was re-used, thus still containing it. It doesn't appear in every episode because different character models did not have the error. People noticed this and called her "Derpy Hooves" from that moment on.
Technically, her original name was Ditzy Doo. She was called this during "Winter Wrap Up" but was never seen. (Lauren Faust has a drawing of her too)
Her derp'd eyes did not start becoming an official thing until "Feeling Pinkie Keen." She was not officially called Derpy yet, but the crew started to use the nickname as well. Later on it was (sort of) announced that Derpy would be used in Season Two as a "Where's Waldo" type of character, and she has been just that throughout several episodes in the season.
Last week, during "The Last Roundup" she was called Derpy for the first time. This made her name and character official through the show. She is still a background character, but she has gotten more show involvement than any other background character.
