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This question deserves to be answered. Lets start a wiki of imaginary saints.
The following saints are from a G.K. Chesterton poem appearing in The Flying Inn and collected in The Collected Poems of G.K. Chesterton under the title Me Heart. Two of the three are imaginary and for whatever reason, the third is St. Barnabas (most likely because of the manner of his martyrdom)
Saint Gallowglass - upon whose eve a lady stole the poet's heart. Gallowglass, is not a saints name, but refers to a group of Irish soldiers which the hero of the story was ostensibly the last of.
Saint Poleander - also upon whose eve a lady stole the poet's heart. Poleander doesn't appears to be anything, but it could be a contraction with oleander, which is pretty poisonous flower.
St. Elsewhere - The nickname for the hospital on eponymous the 80's medical drama. Actually St. Eligius so maybe this doesn't belong here
St. Elmo - From St. Elmo's Fire. Which is apparently a nickname for St. Erasamus. Probably doesn't belong in a list of fake saints since he is also a real person, but find he sounds fake, I thought it worth including.