For a long while, a translation of Numinex's name has eluded us, and we've all seen various theories. Here's yet another, but one I've become fairly convinced of.

Numinex was taken captive by Olaf One-Eye and imprisoned in Dragonsreach. By Paarthurnax's own testimony, Numinex was driven mad and even forgot his own name. Some theories rely on Numinex's name being a corruption or distortion of his true name, and this theory is similar. We have historical evidence of another dragon whose name was adopted and changed by the surrounding culture (Nahfahlaar to the Cyrodiilic Nafaalilargus). My theory on Numinex's original name is:

Nu-Mah-Nax
"The Now-Fallen Cruelty,"
or "Now Falls Cruelty"

This name ties perfectly with his history. Say it ten times fast, and you'll see how easily it would have shifted to Numinex in the hands of the Nords.

The only problem i have here is that the name pretty much fits for a captive that went mad. But that would mean that the dov change names multiple times during their life, always fitting their actual state because Numahnax is surely no name that he would have had prior to his captivity.

That in turn would mean that many dragons named in history could indeed be one and the same as dragons with different names now. And that names the dov have now will be different in the future.

Ofc you could argue that this would be fitting cause of the...if somthing changes it will always have been different -thing but as eternal creatures id more imagine them having a static personality and thus. static names.

Unless dragons use the names we know as "Nicknames" and we have absolutely no idea what a real dragon name is like

Orkar Isber

The only problem i have here is that the name pretty much fits for a captive that went mad. But that would mean that the dov change names multiple times during their life, always fitting their actual state because Numahnax is surely no name that he would have had prior to his captivity.

That in turn would mean that many dragons named in history could indeed be one and the same as dragons with different names now. And that names the dov have now will be different in the future.

Ofc you could argue that this would be fitting cause of the...if somthing changes it will always have been different

This is the case. Consider Durnehviir "curse never dying," who would have had a different name before succumbing to the Ideal Masters. Dragons names can change / be rewritten.

which leads to the question why Paarthurnax is still called Paarthurnax. I didnt have the impression that he is cruel, rather the opposite

Orkar Isber

which leads to the question why Paarthurnax is still called Paarthurnax. I didnt have the impression that he is cruel, rather the opposite

Well, the game calls to question whether his nature has really changed. He may also have "kept" his name as a reminder of the atrocities he has committed.

paarthurnax

Nu-Mah-Nax

"The Now-Fallen Cruelty,"
or "Now Falls Cruelty"

This name ties perfectly with his history. Say it ten times fast, and you'll see how easily it would have shifted to Numinex in the hands of the Nords.

It's my theory that Numinex's name is simply mispronounced by Olaf, or this was Olaf's way of taunting Numinex, as the myths tells us he liked to do. We know dovah names are a shout, consisting of three words. If we break down the three syllables of his name: Num In Ex, we get - Unique Master Ex. ('Ex' not being an actual word in Dovahzul). However, 'Nax', as we know, is a word in Dovahzul, and translates into 'cruelty'. Putting this together we get a sinister name for Numinex: Numinnax - Unique Master (of) Cruelty. So perhaps his name, which Paarthanax tells us he had forgotten in his maddness, was actually Numinnax, but either Olaf (or history) got the name wrong, or Olaf changed this name to torment Numinex, knowing a dragon's name is power, and by bastardizing it, he was removing Numinex's power from him. (figuratively) This also gives the tale of Olaf and the Dragon even more heroics and meaning, since Nimunex is no longer just any dragon, but an important, unique one, caputred and enslaved. (Perhaps as payback for all the 'joor' he forced into survitude.) 

Note that num 'unique' is not canon, however. Numinex's name having changed is still a possibility.

paarthurnax

Note that num 'unique' is not canon, however. Numinex's name having changed is still a possibility.

Being that 'unque' isn't canon, then my theory isn't likely to be the case. Dragons changing their name is a possibility, since no evidence exists either for or against it, other than the assumption that Durnehviir may have. (If not, then he was genuinely unlucky in the name department) But let's assume that it would require a seriously traumatic event to make a dragon want to change his name, since they seem to carry their names with such pride. Numinex would've changed his name in the same format as most other dragon names, three words of power. Which leaves only Olaf to change his name, or his name was lost of the course of history. (and the bards who wrote the tale might not have had any formal instruction in either the language of the dovah, or their nature) All of this really does warrant it's own DLC (which is never going to happen) or a well written quest mod. 

DovahKiinZaan
That would suggest that maybe more than paarthurnax met him, which would mean that more dragons would have gone to dragonsreach to visit him and not kill people. Whilst it's a simple reason to disprove it, it brings up a question: who names the dov?

Dragon names seem to be a title. Much like some Native American names, and perhaps the naming process, or 'naming ceremony' is similar. This could also explain why a dragon's name could change. Take the Navajo. Their names were considered so important, they didn't actually use them in day to day life. They might refer to each other as 'father' or 'son' or 'husband', but reserved saying their actual names for tribal ceremony or in matters of great importance. The tribe gives the names to young Navajo through a naming ritual. I could see dragons being very much the same way, and with Alduin being the oldest and most powerful, he most likely played a key role in the naming of lesser dov. Or it could've a duty delegated to Paarthurnax, which could be why Paar seemed to know Numinex's true name. Maybe from there, each dragon has the choice to change their name (or has it changed for them) as their role within the ranks changes. 

Ceremonial naming at some point(s) of a dovah's life seems like a possibility.

But consider this: After Durnehviir is defeated, he names the hero Qahnaarin (traditionally "vanquisher"). When asked about this, if the dovahkiin questline has not been started, he will say it was his instinct to do so. Qahnaarin fits the dragon name format.

Perhaps dragons just innately know when it's time to change names as part of their attunement to the vensetiid. Then there is the separate issue of retroactivity, of course.

I remember discussing this recently on the Community Wall, and my theory was related to mantling; If you act like a thing and believe yourself to be a thing, you become that thing.

A dragon could name himself Qahnaarin, but if he's not vanquishing anybody, he is not truly Qahnaarin. Once he starts winning in battle, he will become Qahnaarin, and other dragons may recognize him as such.

Conversely, if your name is Alduin 'destroy devour master', and you give up devouring the world for enslaving it, you're going to lose your power and no longer be Alduin.

Whether dragons assign their own names, or are given their names, this still leaves the question of Numinex's unique and unfimiliar name. What we know of Numinex comes from the old tales of Olaf One Eye, a story that in and of itself is a mystery as to what really happened. It's entirely likely the name 'Numinex' comes exclusively from Svaknir's account, which is debatably inaccurate propaganda at best. Olaf was a Tongue, he would've been fimilar with the language and names of the dov. I can't see Numinex taking on this name willingly, even in his madness. Unless of course this was his own way of attempting to 'erase' himself from history out of shame for being captured. But that begs the question, why didn't Durnehviir? He accepted his new role and changed his name accordingly. And realistically, if any dragon was going to go insane from captivity, I would think Durnehviir would've, considering he wasn't even afforded the occasional visit from Paarthurnax to share tinvaak with one of his own kind. 

Perhaps this is meant to remain a mystery. Laan vofahraal.