A Latin professor would say Prium’s the best...
I wish people would stop referring to the latin/greek roots and words. According to normal grammatical rules assuming the Prius is a proper noun (which it is), then you simply add an "s" but since it ends in "s" you add "es". So it is properly Priuses. I have a friend named Gus and an uncle named Gus as well. When I am with both of them I am with 2 Guses. Not Guii or Gusiay or whatever. You cannot under any circumstances change the name of something. If you have a Prii then you have no Priuses.
Octopus -> Octopi
Prius -> Prii (pronounced pree-eye).
That's my story, and I'm sticking to it. I plan on never being anywhere where more than one Prius is present. That way, I will never have to use it in conversation.
All jokes aside, I don't think proper nouns are supposed to be subject to the rules of plurality, and Priuses would be the most common usage.
EDIT: Apparently Toyota actually had a poll about this, and the "offical" answer from Toyota is now prii. It goes along with a big ad campaign about their new Prius models. I'm not sure how I feel about this.
Even if we assume that "Prius" is Latin, not all Latin words that end in -us are made plural by changing the -us to -i. As Cerberus notes, in Latin "prius" is the neuter form of the adjective meaning "before" -- with the masculine and feminine forms both being "prior". The plural of "prius" in Latin is "priora". (Cerberus said all this between his original post and later comment so I upvoted his answer, but perhaps this clarifies a little.)
But just because an English word looks like a Latin word doesn't make it a Latin word, especially when it's the name of a product, which would often be a made-up word.
I'm reminded of the story of the Latin professor who reported to the police that he had been mugged. "Could you identify the hoodlums?" the policeman asks. And the professor pedantically replies, "You mean 'hoodla'."
There is no correct plural for the word Prius at the moment. The matter is still up for debate. It is left to Toyota to determine what this should be. The company has, however, graciously opted to standardize the plural of Prius based on popular opinion. To contribute or make your voice heard on what you think the correct plural form should be, go cast your vote on the Toyota website website. The options are: priuses, prium, prien, prius and prii.
When we pluralize loanwords in English, we can choose to either follow the pattern of the original language or simply follow the English rules. In general, Japanese lacks plural forms (or, to put it another way, every noun is a mass noun) which is why, for instance, samurai is often pluralized as just samurai.
So you can choose:
- A greenwash (the collective noun)
- Prius (follow the Japanese pattern)
- Priuses (follow the English pattern)
- Whatever dumb form Toyota's marketing department comes up with (not recommended)
No plural is needed. Attributive modifiers are always used in the singular and trademarks are attributive modifiers.
Correct: Hand me the Yugoslavia report.
Correct: Hand me the Yugoslavia reports.
Incorrect: Hand me the Yugoslavias report.
Incorrect: Hand me the Yugoslavias reports.
Just as Yugoslavia identifies the country a report is attributed to, Prius identifies the named line of vehicles a car is attributed to. There is only, and can ever be, one such named line.
I suppose one could imagine a case where there were two different lines of cars both called Prius. For example, one can talk about the "Koreas" to mean both North and South Korea. But that would be the only case where one would need a plural of Prius.
Incorrect: I could have had two V8's.
Correct: I could have had two V8 brand vegetable juice beverages.
Correct: Both my wife and I have Prius cars.
Correct: My car is a Prius, and so is my wife's.
Ambiguous: I'm glad I'm a man, and so is my wife.
You can read these, and many other fine rules that no sane person follows, in INTA's trademark usage guide.