The book "Octopus: The Ocean's Intelligent Invertebrate" which I have here says this on the matter: "By the way, the plural of octopus isn't octopi, because the word is Greek -- octopus to be exact -- not Latin. The Greek plural would be octopodes, but we call them octopuses."
You're not being pedantic as it pertains to this word in the English language, you're just wrong.
The word "octopus" comes from Greek, just like the word "corpus" comes from Latin (and has the etymological plural "corpora").
And if it's prescriptive, how far do you want to go, etymologically? According to Wiktionary, the term originates from a Proto-Indo-European language (but doesn't give plural forms for those roots).
And isn't it also the case that once a word gets accepted into a language, it becomes a word in that language, no matter where it comes from? There are _several_ examples of such words, in all the European languages. The word "common", for example, dates back to Latin "communis", and I'm pretty sure the adverbial form for that isn't "communisly", so why the exception for Octopus?
Here's the german conjugation of "mailen" (writing an e-mail), borrowed from "to mail":
Ich maile, du mailst, er/sie/es mailt, wir mailen, sie mailen.
I don't know any loanwords that break english pluralization rules in german, but for the reverse: The correct plural for "Kindergarten" would be "Kindergärten" (not "kindergartens"), which I imagine some english speakers would have problems with. And "Autobahnen" is rather unintuitive compared to "autobahns".
The plural of 'Baby' is 'Babys' (instead of 'Babies' – though some people also use that form) and 'Computer' doesn't change.
On the other hand, both 'Indices' and 'Indexe' are used and for 'Tempus' the only plural is 'Tempora'.
The English word "journal" comes from French, but if you're talking in English about two systemd log files, they're "journals", not "journaux". (If you're talking in French, they are in fact "deux journaux".)