What is the plural of octopus?

Octopuses is standard and preferred, formed by the ordinary English rule of adding -es. Octopi assumes the word is Latin of the second declension, where -us pluralizes to -i as in cactus and fungus, but octopus comes from Greek oktopous meaning eight-foot, and Greek does not work that way. Octopodes is the etymologically correct Greek plural and appears in dictionaries as a rare third option, though using it in conversation is generally a decision about how you want to be perceived rather than about accuracy.

The broader principle is that English does not owe fidelity to source-language grammar for words it has absorbed. We do not say two panini when ordering sandwiches in English, and nobody insists on the Greek plural of chorus. Words naturalize, and the naturalized plural is usually the safest choice. That said, several classical plurals are firmly established and using the English form instead sounds wrong: criteria, phenomena, and analyses are the plurals of criterion, phenomenon, and analysis, and treating criteria or phenomena as singular is a genuine error that appears frequently.

A few adjacent cases follow the same logic. Platypus has the same Greek origin, making platypuses correct and platypi wrong for the same reason. Virus has no classical plural in Latin at all, so viruses is the only option. Data was historically the plural of datum and is now widely used as a mass noun taking a singular verb in general writing, while many scientific style guides retain the plural. Where a word has competing forms, checking a dictionary is faster than reasoning from etymology, since the dictionary reports what educated writers actually use, which is the standard that matters.