This is a speculative question that (I hope) has a good answer from historical linguistics.
My starting observation was that all nouns appear to have a plural genitive ending in -um: -arum, -orum, -(...
In an answer, Draconis said the genitive plural -um (instead of -arum) is sometimes used in the first declension. Now, while -um is fairly common in poetry and with certain specific words, like deum, ...
The 2nd Declension Neuter endings are:
With a word such as auxilium (meaning help, aid), which ...
Some sources mention a genitive singular alius, but I've also seen aliae. And I don't recall seeing a dative singular ali, but neither do I remember alio. I think several forms exist, including even ...
Virus is a neuter word of the second declension even though it ends on -us, as evidenced by its genitive on -i (it has no plural). Are there any other such words?
Bonus question: is it possible that ...