Plural form of words ending in -us
In English, the plural form of words ending in -us, especially those derived from Latin, often replaces -us with -i. There are many exceptions, some because the word does not derive from Latin, and others due to custom (e.g., campus, plural campuses). Conversely, some non-Latin words ending in -us and Latin words that did not have their Latin plurals with -i form their English plurals with -i. Some words' plurals end in -i even though they are not Latin, or that is not the Latin plural, e.g., octopi is sometimes used as a plural for octopus (the standard English plural is octopuses). Prescriptivists consider these forms incorrect, but descriptivists may simply describe them as a natural evolution of language.
Some English words of Latin origin do not commonly take the Latin plural, but rather the regular English plurals in -(e)s: campus, bonus, and anus; while others regularly use the Latin forms: radius (radii) and alumnus (alumni). Still others may use either: corpus (corpora or corpuses), formula (formulae in technical contexts, formulas otherwise), index (indices mostly in technical contexts, indexes otherwise).