There is now more than one type of Prius. So now there needs to be a plural of Prius, for everyone to know and use.
Because Toyota is strange, they have a poll or something on their website that asks you to help them choose a plural. I will present the choices, and destroy them.
1. Priuses. Probably this is the way to go. For the pedantic, "Priusses." It's a logical, English formation. It decides to treat "Prius" as a neologism ex nihilo, but that's fine.
2. Prii. Nope. "Prius" is not a second-declension masculine noun in Latin, and therefore its plural is not "Prii."
3. Prien. Again, no. Pseudo-German, which I assume this is meant to be, makes no sense in this case and that wouldn't be the formation anyway.
4. Prium. What? Seriously, what? That doesn't look plural. That doesn't even look nominative! Stupid!
The actually correct choice, if you're asking anyone with half an education, is "Priora." "Prius" is the neuter of the adjective "prior" (which, yes, means that). Therefore, the neuter plural of this substantive (in the nominative, because we do not inflect our nouns) is "Priora."
That this is ludicrous is not my fault. It is Toyota's fault, for choosing a real word. If you're fussed, I suggest you just start referring to a Prius as a Wooblus, of which the plural, evidently (for Wooblus is a second-declension masculine noun, even if I just made it up), is Woobli.