Octopuses, octopi, or octo?
What is the "proper" plural of "octopus"? A web search turns up three candidates, but is there a "right" answer?
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There is no one "right" answer per se. According to Merriam-Webster, both octopuses and octopi are acceptable plurals.
The reason for the octopi plural is because some people mistakenly associate octopus with Latin words like syllabus and alumnus. Since these words take an -i plural in the original Latin, and since Latin has enjoyed such a high status in the minds of English speakers for such a long time, people have attempted to maintain or reintroduce the Latin plural form for such words.
However, the word octopus is actually of Greek origin. The plural of octopus in Greek is, in fact, octopodes.
Now, I am not advocating using the form octopodes as the plural for this word. English borrows many nouns from other languages (Japanese, Russian, Arabic, and so on) and only rarely ever uses the morphological system of that language to create plural forms. (Even many borrowed Latin -us words do not ever take the -i plural suffix, like circus.)
So, you have three choices:
As I said, there is no one right answer. Good luck to you!
Both octopuses and octopi are accepted alternatives. Octopuses is probably more common and gaining ground since it follows the most common rule for plurals. (The form octopi is the plural form from Latin; the biological genus name Octopus is from Latin, of course.)
Octopuses is the normal English plural.
Octopi is common, but incorrect -- it might be correct if octopus were of Latin derivation, but it is actually from the Greek. Like other abominations such as irregardless, though, it is recorded as a common usage in dictionaries (which are descriptive, not prescriptive).
The technically correct, but vanishingly rare, plural is octopodes.
The most common plural for octopus is octopuses. People with a background in Greek or who want to sound pedantic may use octopodes although it is rarely used. Some people (and dictionaries) also use octopi thinking it is how Latin would do the plural for octopus, but octopus actually comes from Greek not Latin. I don't know if there is a "right" answer since what is right in language comes from how words are used over time. Here is how I would describe the plurals:
octopuses - sounds the least strange and most commonly used
octopodes - technically correct based on etymology, but rare and people may assume you are incorrect
octopi - technically incorrect, but not uncommon