I’ve long wanted it to be octopi but my iPhone and most sources say no. Out of the dictionaries, only Oxford English seems to include but says based on a misunderstanding. Some detail on history here:

https://en.m.wikipedia.org/wiki/Octopus under pluralisation

Interestingly:

> Historically, the first plural to commonly appear in English language sources, in the early 19th century, is the latinate form "octopi"

That’s quite late and interesting the “wrong” one was first.

Tasmanian here.

It’s only platypuses if you see more than one at a time, and that’s pretty much never gonna happen ;)

Alas, I live in Sydney and have never even seen one in the wild. Hopefully someday I’ll get the opportunity to go where they live.

You have undoubtedly heard platypuses more often.

I have never seen nor heard anyone write or say “platypi” before now. Some web sleuthing suggests that platypuses is somewhere between 20 and 100 times more common in English.

Your web sleuthing has an American bias. I'm from an English-speaking country but I'm not American.

Are you from the 1830s? https://books.google.com/ngrams/graph?content=platypi%2C+pla...

Platypuses don’t live in North America. Most of the older sources that Google n-gram is pulling from for the above chart are British or Australian.

A substantial proportion of the recent uses of platypi are grammar guides telling people that it should always be spelled platypuses.