What is the correct plural of octopus:
- Something else?
I would go with octopuses.
That is part of the Wikipedia "Plural form of words ending in -us" article:
- octopuses is the most common form in the UK as well as the US;
- octopodes is rare,
- and octopi is often objectionable.
The Oxford English Dictionary lists octopuses, octopi and octopodes (in that order);
it labels octopodes "rare", and notes that octopi derives from the mistaken assumption that octōpūs is a second declension Latin noun, which it is not. Rather, it is (Latinized) Ancient Greek, from oktṓpous (ὀκτώπους), gender masculine, whose plural is oktṓpodes (ὀκτώποδες).
If the word were native to Latin, it would be octōpēs ('eight-foot') and the plural octōpedes, analogous to centipedes and mīllipedes, as the plural form of pēs ('foot') is pedes.
In modern Greek, it is called khtapódi (χταπόδι), gender neuter, with plural form khtapódia (χταπόδια).
As other answers have already pointed out, the correct English plural is octopuses.
Some clarification is needed in the answers, though, since many people are under the impression that octopi is definitely wrong for historical reasons, while octopodes is apparently acceptable (even though almost no one aside from grammar fanatics has ever heard of it).
After reviewing the historical evidence, both octopi and octopodes have problems.
Aside from the fact that octopi is a familiar plural to many and is indeed listed ahead of octopodes in almost all (if not all) dictionaries, generally following octopuses, there are strong historical reasons why octopodes should be suspect.
Is octopodes "wrong"? No, I don't think so. But arguing for an analogy to native Latin or ancient Greek plurals is misguided in this case. For this specific example, the likely form that the Romans would have used (if they coined the word) could have easily been octopi, particularly if they thought of it as related to polypus at all.
The only way that we get to a position where octopodes is definitely "correct" and octopi is definitely "wrong" is by requiring a word coined in the 1700s to follow rules about ancient Latinized Greek plurals that were very inconsistent in ancient Latin, and are explicitly contradicted by similar words in both ancient Latin and Greek in this case.
I personally would avoid both octopi and octopodes and consider them to be sort of "skunked" plurals. If you want to use octopodes to show off some classical skills, just be aware that, etymologically, you're on somewhat shaky ground.
Octopuses and octopodes are both correct; the former is appropriate modern English, the latter is most appropriate if you're intentionally trying to come off as a pedantic classicist. (I use it regularly.)
Octopoids is the plural of octopoid, not octopus.
Octopi is a mistaken formation based on interpreting octopus as being constructed using a Latin -us suffix when it is in fact constructed using a Greek -pus suffix. (I guarantee you that somebody will write an answer that asserts that it has attained correctness through usage. I contrariwise assert that if ten billion people all jump off a cliff, they're still all stupid.)
(Note: this answer was written for a slightly different question that was merged with the current question, which is why it may seem I was answering oddly.)
The NOAD has the following note about the plural of octopus:
USAGE The standard English plural of octopus is octopuses. However, the word octopus comes from Greek, and the Greek plural form is octopodes. Modern usage of octopodes is so infrequent that many people mistakenly create the erroneous plural form octopi, formed according to rules for Latin plurals.
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