Here's how most words end up on my lists (especially this list): I see a pebble. I like the bizarre shape of it. I like how it feels, and how it sounds when it clacks against other words. I pick it up, dust it off, and put it in my pocket. When I get home to Wordie, I stash it. Many of these pebbles end up on this list of obsolete/obscure/who-the-hell-knows-or-cares-how-they-were-born words. Do I care whether it's a real word? Not a whit. I just found the odd little thing here on this shore of language and thought it was nifty.
Hybridity is hardly a shortcoming in this post-postmodern age, and many modern words are macaronic, e.g. television, cyberspace, and many more. But that does not mean hybridity should be encouraged, especially in such a geezer as mesonoxian, which I suspect was invented by some seicento wit for the purpose of displaying his pseudo-erudition or, perhaps, merely to garner guffaws (not unlike the madeupicals of many Wordies). While I am willing to accept this as a word (not that I have any lexicographic authority in the matter), I would caution against its enthusiastic use, and urge instead the simpler and more elegant adjectival use of the word midnight in its place.
And I should point out to Alsanch1 that crepuscular (a fine word, though one to be used sparingly) has nothing to do with midnight, unless of course one is referring to the the subarctic regions at midsummer); it means "relating to twilight".
Well, to be fair, it does appear on reesetee's list of obsolete words. That there's only one instance in the OED doesn't mean it isn't a word at all. 1623 is certainly more of a pedigree than probably 70% of the madeupical trashwords on this site (and I include my own additions in that statistic).
But since when has being a mutt disqualified a word from existing?
I love the new spelling, sionnach. Are you familiar with brackets, perchance?