Majuu Senshi Luna Varga (1991)

The Dunbas Empire tries to conquer the peaceful land of Rimbell. The three princesses are determined to resist the invaders, expecially the middle one, the tomboyish, sword-wielding Luna. After a magical escapade in the cellars of the castle, she finds herself embedded up to the hips in the head of a huge green reptile, the legendary Varga, and acting as its "brain." Even when in human form she has a reptilian tail. She has to fend off a very persistent giant invader and rescue her sister who has been kidnapped by a pterodactyl. (Source: AniDB)