Off the top of my mind, I know of four hagiographies which contain dragons: St George; St Margaret of Antioch; St Martha; St Lawrence.

Much like in Revelation, the dragon in these tales is portrayed as evil, and their defeat herald as a sign of divine righteousness. In this regard, it is almost in lock-step with the allegories seen in non religious texts. Although there is a touch romanticism in relationship to the dragon and the female saint and her virginity.

I would have given a far better answer if I wasn't so stuffy headed and sick, but the following books are great on the subject:

A Companion to Middle English Hagiography by Sarah Salih

Beowulf and the Dragon: Parallels and Analogues by Christine Rauer

The role and symbolism of the dragon in vernacular saints' legends, 1200-1500 by Patricia Brown (Ph. D thesis)