Derpy's the essence of what the brony fandom is. Without it, Derpy would never have existed. Inspired by a simple animation error, she became the fandom's beloved mascot.

There are two great videos that capture her beauty perfectly:

Derpy Loves Her Lava Lamp:

Derpy's Christmas:

Also, Forest Rain's signature song: Great to Be Different.

Happy Derpy Day, everypony! /)