What is the plural of Prius?

I suspect that Toyota's advertising department just made up a word they thought sounded nice. If it were Latin and a noun/adjective, the plural would be priora. Since this probably is not the case, Priuses seems to be the only choice.

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'."