Dictionary.com claims that the plural of 'apparatus' is 'apparatuses'. Surely that can't be right... isn't it 'apparati'?
The Latin plural of the noun apparatus is actually apparatus. (Sometimes the Latin is spelled singular apparātus and plural apparātūs; the vowel lengthens in the plural, but that's not usually reflected in the spelling.) This is because it's fourth declension. In the 18th and 19th centuries, when most educated English speakers had studied Latin, apparatus was sometimes used as the plural; I believe this usage is quite rare today.
With status, apparatus, and other Latin nouns of the fourth declension ending in -us, the English plural adds an -es. The Latin nouns which pluralize by turning an -us into an -i, like radius, fungus, alumnus, are all second declension. For some Latin nouns of the second declension, like campus, the Latin plural has been lost, and the English plural adds an -es. But campi would be correct in Latin, while apparati is incorrect in both English and Latin.
You don't actually hear the English plural apparatuses that often, because apparatus is often treated as a semi-uncountable noun: one apparatus, two pieces of apparatus. (See Google Ngrams.) I assume this usage developed because people didn't know whether to use apparatus or apparatuses for the plural, and figured out a way of avoiding the issue altogether.
To write "apparati" is an example of a hypercorrection:
In linguistics or usage, hypercorrection is a non-standard usage that results from the over-application of a perceived rule of grammar or a usage prescription. A speaker or writer who produces a hypercorrection generally believes that the form is correct through misunderstanding of these rules, often combined with a desire to appear formal or educated.
It's the pedant's own-goal, basically. I've made many. Using "whom" as a subject pronoun is a classic.
Some more examples similar to the question are:
- octopi instead of octopuses
- scenaria or scenarii instead of scenarios
- penii instead of penises
Although in some cases (see "octopi" vs "octopuses" on Ngrams) the hypercorrected form may be common enough to be accepted as standard usage.
In the case of "syllabus", for instance, an analysis of the Latin shows "syllabi" to be a hypercorrection. However Ngrams shows it to be more widely used than the supposedly more correct "syllabuses".
As @JohnLawler said, not all Latin words keep their Latin plurals. Consider:
- virus/viruses (not virii, see link)
- status/statuses (yes, statuses)
- stadium/stadiums (Greek word actually but came to English through Latin and (almost) nobody says stadia)
And the list goes on. I have no Latin so I don't know how many of these don't keep their plural because of the rule mentioned in @PeterShor's answer but you can see that many Latin words have normal English plurals. In other words, trust the dictionary, it tends to know more than we do.
This singular/plural problem affects many languages that have given loanwords to English. Arabic is a bugbear for non-Arabists. The word Caliph (originally khalifa) has its correct plural as khulafa'. An increasingly common term, hadith (a prophetic tradition) is ahadith, and that confuses because of what looks like an English indefinite article at the beginning. (The middle vowel also lengthens. A Bedouin is worse: it is already an inflected sound plural (the previous two are 'broken') and it is also used in English as a plural. One Bedouin is a badu, nominative Bedouins are Badawun, and the adjective is badawi. A similar form is assassin, which starts as an inflected plural hashshashin. A hashshash is, reputedly, someone who smokes hashish (grass). Hummus comes from a collective plural himmas (and variants),chickpeas. And much more....