Why is octopi the plural of octopus but platypus is platypuses and not platypi?

Answer 1

The pluralization of octopus as "octopi" is in fact, in its Latinate form, incorrect, as this would require it to be a second declension Latin word. Since its classification in Latin form is third declension, according to classical Latin grammar rules, the plural of octopus should be octopodes.

In any case, the more commonly used plural of octopus is "octopuses" - which classicists regard as an incorrect oversimplification.

Besides this, the plural of platypus cannot be universally agreed upon either - "platypuses", "platypus", and "platypi", as well as the grammatically correct plural "platypodes", are used with varying levels of popularity.