In the octopus' garden in the shade

Today, my mom, who is pretty damn smart in addition to being the prettiest lady in the world, told me something I didn't know. Apparently, the plural of octopus is not octopi. It's octopuses. The reason is that octopus doesn't come to us from Latin, it comes from Greek, from okto, meaning eight and pous, meaning foot. The Latin word for octopus is polypus, meaning many feet, even though the Latin word for foot is pes and not pus. In Greek, the plural of octopus is octopodes. Which makes me wonder why we so often use the Latin pluralization for Latin words when we don't usually use the Greek pluralization of words. 
And come to think of it, there are tons of words that come straight from Latin that we don't use the Latin plural for - the plural of status isn't stati, virus isn't viri, the plural of iris isn't iri (especially since the plural for iris in Latin is ires).
My favorite pluralization fact, which I've mentioned here before: a boy who graduated from a high school is an alumnus of that school, where a girl is an alumna. A group of people who graduated from a high school are alumni, unless every member of that group is a woman, in which case, they are alumnae. However, if even one member of a group of alums is a boy, the whole group becomes alumni. Which makes absolutely perfect sense in Latin.