Yeah, apparently you need to know the origin of the word to know how to pluralize it.

One octopus, two octopuses. But you can also say "octopodes" because the elements used to create the word are originally Greek (okto for 8, pous for foot), and that's how Greek words get pluralized. But, although it was based on Greek elements it was never used in Ancient Greek. It was a modern Latin word, created in the 1700s as a scientific term using those Greek elements. As a Latin word, the "us" ending should be pluralized with "i", so "octopi" (which is one of the oldest known pluralizations of the word). But, it's an English word, and the proper way to pluralize an English word ending in "us" is to tack on "es".

So, you can go with "octopodes", "octopi" or "octopuses" and have an argument why any of them is correct.

For Unix, since it's a word created in English, it's probably "unixes". To claim it's "unices" you'd have to pretend that "unix" is a Latin word, which it isn't, and never was, but "ix" is a common declension pattern in Latin, and an uncommon ending in English, so it's fun to pretend it's a Latin word and doesn't get pluralized normally.