Derpy Hooves is the fan-given name of a gray Pegasus pony.

Derpy Hooves is the fan-given name of a gray Pegasus pony. The 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 less wall-eyed.

http://mlp.wikia.com/wiki/Derpy