Derpy Hooves is a gray Pegasus pony whose name was given by the show's internet following due to her having a cross-eyed "derpy" expression in the first episode. The name has since been adopted by the crew and Hasbro. Starting with Feeling Pinkie Keen, the show's crew began giving the pony crossed eyes intentionally, and later gave her "Where's Waldo" type cameo appearances, as a nod to the fans. The character is addressed by Rainbow Dash in The Last Roundup as Derpy, where she speaks for the first time. the scene was later altered so Rainbow Dash no longer names Derpy, her voice was changed, and she is a little less wall-eyed.
In Applebuck Season, Derpy is seen saying "Mmmm, muffins!" in the backround.
In Griffon the Brush Off, she is seen standing in front of the vegetable stand.
In Boast Busters, Derpy is seen standing in the crowd for Trixie's show, flying above the crowd, back down on the ground, in the crowd looking at Twilight, then finally glaring at Snips and Snails.
In Look Before You Sleep, she is seen taking down a loose branch.
In Bridle Gossip, Derpy isn't seen.
In Swarm of the Century, she is seen flying in the park.
In Winter Wrap Up, Derpy is seen in the crowd wearing a blue jacket listening to the mayors speech. Later she is flying up to the weather, then she's seen kicking clouds with Parasol. Then, she's seen blowing away clouds with Parasol and Medley, and later spiraling up into the sky with them. Finally, Derpy is seen standing in the crowd looking up at the mayor and Twilight.Fall Weather Friends, Derpy first appears watching the iron pony competition and dropping a rose from the clouds. Soon after, she's watching the football portion of the competition. Then, she's seen holding the banner for Rainbow Dash. Later on, she watches the start of the Runnning of the Leaves. She is then seen standing by the finish line. Finally, from the foreground she watches Rainbow Dash and Applejack fight.
In Suited For Success, She is seen walking towards Rarity's fasion show from the bottom left. Then, she is shown looking at Hoity Toity as he walks up. Next, we see her in the crowd watching the models go onstage. Finally, she is seen standing behind Bon Bon.
Derpy Hooves originally started out as a background pony but quickly became a fan favorite due to her easily identifiable derpy eyes. Since her first appearance, many fans have made or written pictures and stories about her. Derpy's fandom-originated name being used officially in the show sparked massive joy and excitement amongst the fans.
Derpy's name is officially used in The Last Roundup and Derpy herself speaks her first lines in that particular episode. However, the scene has since been altered in the sense that Derpy's name is no longer mentioned by Rainbow Dash, Derpy's eyes are fixed for the majority of the scenes she appears in (instead of being crossed), and Derpy's voice has been changed (originally, she was voiced by Tabitha St. Germain). A comparison of the changes of the scene can be viewed here.
Fans of the show have mixed feelings about Derpy's new voice (some stating they prefer it due to it being more feminine while others state they prefer the old voice because of its originality). However, most fans across the world dislike of the removal of Derpy's name and eye-crossing in her scenes of The Last Roundup. The reason for this change can be found here
- In Winter Wrap Up, a Pegasus, named Ditzy Doo (possibly Derpy) went to grab the birds from the north instead of the south. Soon after hearing this information, Rainbow Dash states that the previous year she went west to get the birds. Fans have speculated about Ditzy Doo being Derpy's official name. Lauren has even gone and said "If Ditzy is ever used again, we will likely assign Derpy's model to her."
- The name Derpy Hooves has its origin in the fandom, but has been adopted to official status by the show crew. Judging by the sudden alterations to Derpy's scenes of The Last Roundup, it is possible for one to assume that Derpy's name has been uncanonized (meaning that Derpy Hooves is no longer known as 'Derpy Hooves' in the My Little Pony universe).