[Latin or Italian plural endings.]
Some Latin nouns ending in -us that form their plurals in i usually retain that plural form in English: bacilli, fungi, gladioli, narcissi, stimuli. Many, however, have adopted English plurals completely (viruses), or can use either (foci or focuses; radii or radiuses).
Other examples are plurals of Italian nouns, either those ending in -e (cognoscenti, dilettanti), or in -o (bambini, divertimenti). Some Italian plurals for things that are not individually counted have become mass nouns, grammatically singular (confetti, macaroni, spaghetti); at one time graffiti seemed to be going the same way, but the singular graffito is now often encountered. Some examples are acknowledged to be plurals but their singulars are uncommon (biscotti).
Other English nouns ending in -i derive from other languages: Japanese (origami, sushi), Arabic (alkali), Urdu (biriani), Hindi (basmati).