I took my children to the cinema on Tuesday, and was treated to an advertisement - this one - soliciting views on the appropriate plural form of the car name 'Prius'. By way of illustration, the advert asserted that the plural of 'Octopus' is 'Octopi', but the plural of 'Platypus' is not 'Platypi'. Which is strange, because the names mean "eight-footed" and "broad-footed": in both cases the ending is from Greek πους. The classical plurals would be 'Octopodes' and 'Platypodes', and the (sensible) English plurals are 'Octopuses' and 'Platypuses'.
The name 'Prius' would seem to be a Latin neuter comparative, meaning "earlier". If so, the classical plural would be 'Priora'. And the sensible English plural would be 'Priuses'.
I'm faintly surprised to learn from Toyota's website that voting is long since closed - did they show us the ad by mistake? It seems hoi polloi were invited to choose between 'prii', 'prius', 'priuses', 'prien', and 'prium', and the plurality vote was for 'prii'. My plan is to avoid any further encounter with this plural form, by means of not buying the things.