<<Even if the Latin plural were known, English speakers would not be obliged to use it. Examples of Latin loanwords into English which have regular English plurals in -(e)s include campus, bonus, anus and cancer. These stand beside counterexamples such as radius (radii) and alumnus (alumni). Still other words are commonly used with either one: corpus (corpora, or sometimes corpuses), formula (formulae in technical contexts, formulas in non-technical usage).
As a word in Botanical Latin, cactus follows standard Latin rules for pluralization and becomes cacti, which has become the prevalent usage in English. Regardless, cactus is popularly used as both singular and plural, and is cited as both singular and plural. Cactuses is also an acceptable plural in English.
There are three plural forms of octopus: octopuses, octopi, and octopodes. Currently, 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 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 octōpedēs, analogous to centipedes and mīllipedēs, as the plural form of pēs ('foot') is pedēs.
The situation with the word platypus is similar to that of octopus; the word is etymologically Greek despite its Latinized ending, and so pluralizing it as if it were Latin (i.e. as platypi) is ill-considered. As with octopus, importing Greek morphology into English would have platypodes as the plural, but in practice this form is not well-attested. In scientific contexts biologists often use platypus as both the singular and plural form of the word, in the tradition of sheep or fish, but laypersons and scientists alike often use the simple English plural platypuses. There is no consensus on which of these two is correct, with different dictionaries making different recommendations.
The English plural of virus is viruses. This is non-controversial and speakers would not attempt to use the non-standard plural in -i. The Latin word vīrus (the ī indicates a long i) means "1. slimy liquid, slime; 2. poison, venom", denoting the venom of a snake. Since vīrus in antiquity denoted something uncountable, it was a mass noun. Mass nouns—such as air, rice, and helpfulness in English—pluralize only under special circumstances, hence the non-existence of plural forms in the texts. It is unclear how a plural might have been formed under Latin grammar in ancient times if the word had acquired a meaning requiring a plural form. If vīrus were a masculine second declension term like alumnus, it would be correct to use vīrī as its plural. However, it is not a masculine second declension term, as was explained. There does exist a Latin word virī, meaning "men" (the plural of vir, a second declension masculine noun), but it has a short i in the first syllable.
Facetious mock-erudite plurals in -i or even -ii are sometimes found for words ending with a sound (vaguely) similar to -us. Examples are stewardi (supposed plural of stewardess) and Elvii (as a plural for Elvis imitators). The Toyota corporation has determined that their Prius model should have the plural form Prii, even though the Latin word prius has a plural priora, the Lada Priora having prior claim to that name—though the common plural is "Priuses". The Winklevoss twins are sometimes collectively referred to as "the Winklevii".>>