[Battle Axe]

Because it is Latin. And the Latinspeakers made up all their own rules.

[G8 Diplomat]

Because it's Latin. And the Latinspeakers make up all their own rules.
Yes, and in Latin most words that end in -us in the singular end in -i in the plural. In fact, the Latin plural if virus is viri, but it didn't stay like that in English, unlike cactus, cacti and radius, radii.