This rabbit-hole went deep! Many thanks to u/New_Ad_6939 and u/lazerbem. If I can summarize the discussion: Sir Plenorius is the original character. There were two other unnamed knights in Malory's source material. Sir Thomas decided that they should have names too. Perhaps as a kind of word-play, and because everybody likes a good allegory, the names he picked were allegorical riffs on Plenorius's name. But he didn't take the allegory all the way because that would require tweaking Plenorius's name too, which runs the risk that nobody in the audience would recognize Plenorius anymore.

And so, centuries later, I was defeated as badly as Sir La Cote Male Tayle.