There are two words in American English that are commonly misused in the plural forms.
One is “alumni.” The Latin word for “student” is “alumnus.” Its plural form is “alumni.” One alumnus, many alumni. Ironically, the misuse is often even propagated by university alumni organizations which refer to its members as alumni (correctly) but also sell bumper stickers and other paraphernalia with labels like “Alumni of SoAndSo University.” You are actually an alumnus of SoAndSo University.
Another is “octopuses.” The word comes from Greek, and the plural form is “octopodes.” The Latin word for “octopus” is actually “polypus.” There is no “i” in any form of octopus.
And that’s my language lesson for the day.