Learn English – the plural of “equipment”

This lack of respect for the language of origin not a phenomenon unique to English. When a word is borrowed into one language from another, unexpected things can happen.

I would argue that, for many examples you've given in your question, the actual perception of a singular-plural relationship is messy in practice, and the application of the plural is inconsistent.

Data: Using data as a collective noun with singular agreement is more common than using it with plural agreement. More in another thread from this site.

Alumni: I have heard as many people also use alumni for the singular, or even alum, as I have heard use alumnus for the singular. I imagine my experience with this word is typical (at least in the US), though certainly not universal. In any case, it is messy.

Media: The words media and medium don't even seem to correspond in any meaningful way in actual English usage. The word media has forked off and become a different word entirely. The word media is clearly used as a collective singular noun, as shown in newer constructions like multimedia (not multimedium even though we don't say multistages, multicores, multicycles, multistories, etc.). You will find few people who will ever say "Mass Medium". We talk about someone having "media savvy" even though we wouldn't say "computers savvy" (even though they can work with more than one computer). This is because, in English, these sorts of constructions always use the singular noun, whether it is collective or not. The way that media is used is evidence of how the word is actually parsed, perceived, and used by English speakers.

Another example of how foreign language morphology often doesn't mesh well: people try to pluralize octopus and virus as octopi and viri/virii, respectively. Virus was a mass noun in Latin, where we got the word. The word octopus comes from Greek and would take the plural form octopodes in Greek.

My main point is this: there is only a weak, inconsistent application of this -us to -a or -us to -i to begin with. So forums (like statuses and others) is a word even though we also sometimes have this other rule. Our language seems to continually push us towards either dropping the foreign pluralization in some way or another, or reanalyzing the plural as another distinct word. So I see this confusion as the language trying to mash these words around to make them fit our language naturally.

If we hadn't become so darn literate and knowledgeable in the past few centuries, I imagine these plurals would have regularized by now :)

Merriam-Webster's Eleventh Collegiate Dictionary (2003) confirms the derivation that appears in medica's second comment above:

quota n [ML, fr. L quota pars, how great a part] (1618)

The Eleventh Collegiate's entry for quota doesn't include a plural form of the word, but that indicates that the plural form is simply quotas, because Merriam-Webster's typically includes a word's plural form only when that form varies from the standard English form of -s (as in the entries for quality ["n, pl -ties"], quantum ["n, pl quanta"], and sheep ["n, pl sheep"]) or when multiple plural forms are in fairly common use (as in the entries for agendum ["n, pl -da or -dums"] and zero ["n, pl zeros also zeroes"]) or when the plural might be mistakenly rendered as -es instead of -s (as in the entry for solo ["n, pl solos"]).

Bryan Garner, Garner's Modern American Usage (2003) offers this useful general advice on how to deal with plurals of "borrowed words":

Words imported into the English language from other languages—especially Greek, Latin, French, and Italian—present some of the most troublesome aspects of English plurals. Many imported words become thoroughly naturalized; if so, they take an English plural. But if a word of Latin or Greek origin is relatively rare in English—or if the foreign plural became established in English long ago—then it typically takes its foreign plural.

One reliable guide is this: if in doubt, use the native-English plural ending in -s. That way you'll avoid the mistakes involved in hypercorrection, which is rampant with false foreign plurals (as when people say or write ignorami instead of ignoramuses, thereby betraying something quite ironic). ... Many writers who try to be sophisticated in their use of language make mistakes such as ignorami and octopi—unaware that neither is a Latin noun that, when inflected as a plural, becomes -i. The proper plural of the Greek word octopus is octopodes; the proper English plural is octopuses.

Garner acknowledges the existence of various exceptions (such as crises, criteria, and phenomena) and "extremely close calls" (such as cactuses/cacti and millenniums/millennia). But in the long run, he argues, a grandiose notion of fidelity to the Latin or Greek sources of English borrowed words may serve writers ill:

It's pedantic and prissy to say that politicians attend fora, enter auditoria, ascend rostra, and speak in favor of referenda.