"In order to enjoy the comforts of swearing without incurring the penalties of profanity, the French invented a calendar of fictitious saints' names to swear with—St. Lache, the patron of idlers; St. Nitouche, who watched over hypocrites; and St. Gris, beloved of drunkards—to which the ribald Rabelais adds a medley of his own: "By St. Godegran, stoned to death with apple dumplings . . . by St. Foutin, the fornicator's friend! . . . by St. Vitus and his jig! . . . by St. Mamica, the virgin martyr, by our lusty mammical duty to all virgins!" (William Iversen, "O the Times! O the Manners!").  [Thanks, Jonathan!]

But here's where the concept of "fictitious saints" becomes really interesting:

"Saints, as extensions of a corporate and totalitarian pseudo-religious regime, are always false and never actual in the sense that there is nothing magical or divine about them. They are mere men and women, and often quite evil men and women. But, even more interestingly for those of us interested in paganism, saints were sometimes not even based on actual people, but were simply made up like characters in a novel to sit astride the previous pagan tradition. Many saints simply do not have any historical or biographical basis. Such saints represent a mere renaming of pagan deities. These falsest of the false saints, in other words, are hidden pagan gods" (Colin Liddell).

We would take this idea a bit further: when a saint—being a fictitious sort of entity in the first place—is based not upon an actual person but upon an imaginary character, then we've stumbled into the realm of genuine mythology.