Dragons can be found on folios 23r and 32r, during the portion of the manuscript dealing with the Office of the Dead (see the first two images, left and below). These creatures were associated with the Devil in the Middle Ages, which may be one reason why two dragons decorate initials for the Requiem chant during the Office of the Dead. According to T. H. White's translation and edition of a twelfth-century bestiary,
“DRACO the Dragon is the biggest of all the serpents, in fact of all living things on earth…The Devil, who is the most enormous of all reptiles, is like this dragon. He is often borne into the air from his den, and the air round him blazes, for the Devil in raising himself from the lower regions translates himself into an angel of light and misleads the foolish with false hopes of glory and worldly bliss. He is said to have a crest or crown because he is the King of Pride, and his strength is not in his teeth but in his tail because he beguiles those whom he draws to him by deceit, their strength being destroyed. He lies hidden round the paths on which they saunter, because their way to heaven is encumbered by the knots of their sins, and he strangles them to death. For if anybody is ensnared by the toils of crime he dies, and no doubt he goes to Hell.”
T. H. White, ed. and trans., The Book of Beasts: Being a Translation of the Twelfth Century (New York: G. P. Putnam’s Sons, 1954), 165-167.
Dragons were also associated with childbirth because St. Margaret, the patron saint of childbirth, was believed to have escaped from the belly of a dragon unhurt. Since this "S" of "Suscepimus" on folio 3v begins in the final chant of the Feast of the Purification, it is possible that the scribe was using the dragon to emphasize the fact that this feast was the time when women could re-enter church after giving birth.
On the other hand, this initial reminds us that it is always possible that animal and human imagery was included for aesthetic reasons or simply because the scribe simply wanted to doodle. Indeed, the dragon in this image certainly has a quality of doodling rather compared to the planned-out, cadelled brown and yellow initials we see elsewhere in the manuscript.