Melkor, “he who arises in might”
> Morgoth, “black enemy”, was given to the evil Vala by Fëanor in his grief and anguish.
Fëanor was the chief of the
Lambengolmor, “Loremasters of Tongues”, a Noldorin school of study he founded. (Rúmil and Pengolod would presumably have been members of this group of scholars.) The shift
Melkor > Morgoth, (Q:
Morikotto) both consonates and alliterates, particularly in Quenya. It is a not-so-subtle intellectual insult, and with a little effort, we could probably come up with a short list of real-world names similarly and deliberately mangled. In any case, it might be “a low philological jest”, which as David Salo (a better Tolkien scholar than I) has recently suggested “
is not at all untypical of Tolkien’s linguistic work.”
Elves were particularly fond of language. If we grant for a moment (even in an imaginative stretch) that
Mairon was the Sindarin version of Sauron’s original name in Valarin, and that he used it in dealing with the Elves in Middle-earth, the shift
Mairon, “Admirable” >
Þauron “Abominable” >
Sauron both alliterates and rhymes. “Regular elvish trick,” as Gorbag might have put it. That might also explain why there was an older name for Sauron in Sindarin,
Gorthaur, “abominable fear”. Perhaps the
gor- was simply dropped, or maybe the Eldar in Middle-earth punned his preferred name.
In both cases we can see alliteration. In Quenya, second similarities can be found at the beginnings of the syllables (consonation), while in Sindarin the second similarities are shifted to the ends of syllables (rhymes). That would fit Tolkien’s style of differentiation between the parent and daughter languages, Quenya and Sindarin: similar forms, but not quite the same. It would at the same time highlight the close relationship between the thought processes of the Calaquendi and the Sindar, the Amanyar and Úmanyar.