As does dictionary.com, although they wimp out and give both hippopotamuses and hippopotami as plurals.
Technically, the Greek plural of hippopotamus is hippopotamoi.
Don’t let anyone try to tell you I didn’t learn anything from my Greek major.
I just want to point out that datums is, indeed, correct in certain contexts.
I just remembered another one - “Rhinoceros” It’s not even a “us” word, but how many times I’ve heard it as “rhinoceri”… even in print.
Yes indeedy! I always call them “forums”, and I WORK here. I knows better but I does it anyways.
Oooo… poor didda 
Actually, I like Greek and Latin plurals usually, but I don’t like “fora”.
If we agree that language is evolving, and “right” or “wrong” ways of using it are based on consensus, then we can accept that many words of Greek and Latin origin have lost their original plural form to the English ‘s’ and ‘es’, whilst others have retained them. This is an arbitrary process. The problem with words like “forum” is that the process is half-complete. I doubt anybody will use “fora” fifty or a hundred years from now, and “forums” probably would not have been used a century ago. The word’s half-evolved status means that whichever plural form you decide to use, there’ll be somebody who won’t like it.
For mine, I’ll stick with “forums”. It sounds better. “Fora” does sound a bit pretentious to me.
Errr… although I once posted the Latin plural of “forum”, and therefore bear some responsibility for the prevalence of this trend… I’m inclined, personally, to use the English word “forum”, whose plural is “forums”.
It’s important (well, if you’re a grammar freak with A-level Latin, it’s important) to remember that “fora” is only the nominative plural of “forum”. If you feel obliged to use the Latin plural form, you should feel obliged to use it correctly… and Latin has case inflections.
So, you can say “These fora are full of fascinating material”, no problem. And the accusative form is the same, so you can add “I like these fora”. But the dative and ablative form is “-is”, so you’d have to say “I contribute regularly to these foris” and “My life is being taken over by these foris”. As for the genitive “I enjoy being a member of these fororum”, the less said about that the better.
Where this leaves us with data, media, mediums, and hippopotamuses, I really couldn’t say.
rhinoceroi
Rhinocerotes
(Prepoceros!)
Yes, that is correct. In Haiti.
Some collective plurals:[ul][]crash/herd/bloat of hippopotami[]crash of rhinoceri[]murder of crows[]shrewdness of apesarmy of me[/ul]
“Fora” wasn’t a word I’d heard used for real until law school. We do like our latin and we do take it fairly seriously, even if our pronunciation is completely bollixed…anyway, fora’s useful when you’re discussing jurisdictional issues.