The Dragon is one of the main characters from Dreamworks' 2001 computer animated film Shrek. (Beware of spoilers, especially below the line.) The following blurb is HOCR from the movie's offical site,

Even though it breathes fire and eliminates any knight who tries to save the princess, this is not your ordinary fairytale dragon. Hanging out at the Keep and being surrounded by hot boiling lava all day long can make any dragon hungry for a little action.

The Dragon is quite a clever twist on the traditional dragon-guarding-the-princess role. The Donkey sweet-talks Dragon, giving Shrek time to rescue the princess, but does too good a job -- this very feminine female dragon is smitten with the donkey, head over heels. The Donkey is not comforable with this relationship, and is rescued from reverse dragon rape by Shrek, just in the nick of time, of course. The princess rescue sequence is one of the more clever acts in the movie.

The Dragon plays a part in delivering the "everyone is a good person no matter how they look" moral of the story: she and the donkey become fast friends by the end of the film.