The plural of octopus
Octopuses, octopi, or what?
English, like most languages, includes many loan words (or borrowings) from other languages. In most cases, when these words are nouns, the plural is formed with the usual -s. For example, pizza/pizzas (not pizze), kindergarten/kindergartens (not Kindergärten), ninja/ninjas, guru/gurus, typhoon/typhoons, and so on. But for some loans, especially from Latin, English also borrows the original plural.
A common class are Latin second-declension masculine nouns in -us, with plurals in -i:
fungus → fungi
alumnus → alumni
radius → radii
locus → loci
Not all such words use the Latin plural in English. For instance, campus and genius take the usual English -uses instead of the Latin -i. And for many words either form is acceptable.
Nor are all English loans from Latin in -us of this class. Third-declension neuter nouns in -us take a plural with -era/-ora:
genus → genera
corpus → corpora
opus → opera
Again, the English plural in -uses can often also be used.
A large class of Latin fourth-declension words in -us pluralize by lengthening the u. Since this has no phonological analog in English, -uses should be used. Examples are status, nexus, hiatus, and apparatus.
In general, Latin loans in -us other than in the first two categories should use only the English -uses. (A difficult case is virus, an unusual word for which there is no record of a Latin plural.) And some words in -us (such as caucus) are not even from Latin.
And what of octopus? It was not a classical Latin word at all but coined by Linnaeus around 1730. It may seem wrong to use a (pseudo-)Latin plural in English with an invented word. English dictionaries list octopuses as the preferred, and sometimes the only correct, form.
However, one could also argue that (1) medieval Latin as the common tongue of educated Europe was a living language and still Latin even with new words, and/or that (2) since octopus was coined from Latin parts, using Latin forms is justified, much as if I coined a scientific term millifungus, its plural should still be millifungi.
The word octopus was built on analogy with existing -pus (=foot) words in classical Latin, particularly polypus, which is what the Romans would have called an octopus. However, these words in Latin were in fact loan words themselves, from Greek! And, just like English, in Latin borrowings from other languages sometimes borrowed the plural and sometimes used a regular Latin plural. And, again like English, in some cases either one could be used.
When any Greek plural was borrowed, commonly a “Latinized Greek” variant of the third declension was used, in which the plural of octopus would be octopodes (patterned like antipodes). This is what Linnaeus used. Some dictionaries list it as an acceptable form in English, but most people would probably consider it affected.
Alternatively, some Greek words had a regular Latin plural, and in fact the plural polypi is recorded. On this basis, octopi, often disparaged as erroneous, has some justification. However, in English usage it originates not from any stage of Latin but by English speakers applying the -i plural indiscriminately.
Meanwhile, the form octopuses is undisputed and almost universally considered primary.