Helloes!

Lately I've been very annoyed by the octopi/peni(i)/whatever-i fad. English isn't my first language, and I don't know Greek or Latin, but I still don't think these are the correct plural endings. With what words is this form correct? For borrowed words, is normally the forms from the language the word originated used, or the English forms apply?

In my language, we use 'i' endings for certain declensions of masculine words, but if these words were borrowed into the English language, I'd rather see them in singular form + s/es. It just doesn't seem well incorporated in the sentence, otherwise. Though I have no way of backing up my claims.

Someone tell me if this rant is justified or not, and I'll calm down. xD
Thanks!