Hi Davide, There are some English words which end with the letters "us". Examples are: radius / syllabus / octopus / cactus. These are singular nouns. When we use the plural forms of these aforementioned words, they become: Radii (or Radiuses) / Syllabi (or Syllabuses) / Octopus (or Octopi) / Cacti (or Cactuses). The "us" is replaced with "i". Alternatively, "es" can be added to the singular noun. As Darryl rightly pointed out, there are many exceptions to the rule in English. Many singular nouns that end with "us" cannot be transformed to the plural form by using "i". Example: Bus --> Buses (right) / Busi (Wrong) Campus --> Campuses (right) / Campi (Wrong) Hope this helps. Cheers, Lance

From what I read online, both are fine. In casual speak however, people tend to want "octopus" even when referring to its plural form. (yeah… I know. But saying "Octopuses" is too long and clumsy for some) There are way too many exceptions in English, so just do what everyone else in love with the language will do: just accept it for what it is, quirks and all.

https://en.wikipedia.org/wiki/Octopus#Etymology_and_pluralization