Why is she riding a dragon?
In legend, St. Martha journeyed to France where she tamed a dragon by prayers and hymns. This dragon was called the Tarasque, and was traditionally pictured as having six legs, a spiked turtle-like shell, and a man's head, but was sometimes pictured as a more traditional type of dragon. The Tarasque was killed by villagers, who, after they were converted by St. Martha, in remorse renamed their town after the beast. The Tarrasque in D&D is named and designed after it.
Thanks. I ended up writing a post of my own about the topic.