English is pretty casual about borrowing other languages' plural forms, and Latin second declension nominatives -- alumnus/alumni, millenium/millenia -- are probably the most familiar non-English singular/plural pairs to English speakers. To an English speaker, Prii seems like a reasonable plural for Prius (though if it's a Latin word, it looks like a neuter comparative, in which case the plural is "priora"). Similarly, "Lini" looks like the plural of "Linus". It's an interesting philosophical question: given that English speakers are familiar with this Latin pattern, can apply it spontaneously to new words, and perhaps are even unaware that it comes from Latin, has this become a backwater grammatical rule of English? We're willing to say originally French words like apartment and address are English. How about Lini as the plural of Linus?
Afaik it's only with Latin that English-speakers insist on universal adoption of the foreign plural. For other words the irregularity is a rare exception―while for Latin it's pretty much regularity instead.
For some reason, people don't seem so keen on using French plural forms for French words: e.g. "animaux" and "journaux." Same with German ones, Italian, Yiddish or Arabic. Probably because borrowed words are mangled to the rules of the adopting language, like it's done everywhere.
IMO not only using Latin plurals in English doesn't make sense and is rather arbitrary, but it's kept around by people insisting on correcting others contrary to those others' intuition. It's time to stop.
