The Suns of Tyranny’s “On The Road Again” features the line “Apocalypses come and go”, so that seems authoritative.
Short, stocky bearded miners are always “dwarves”. Small main-sequence stars, especially those of K or M spectral class, are always “dwarfs”.
There’s also “antennas” vs “antennae.” The former seems to only refer to the devices that transmit or receive electromagnetic signals, while the latter applies to the protuberances on insects. I feel like there’s a few others of these English vs Latinate plurals where the English plural refers to the every-day object, and the Latinate refers to a more specialized, often scientific, meaning.
Riley Finn, is that you?
Riley: But you killed the… You did the thing with that… You drowned! And the snake?! Not to mention daily slayage of… Wow.
Buffy: It’s no big, really. Hey, who wants ice cream?
Riley: Buffy. When I saw you stop the world from, you know, ending, I just assumed that was a big week for you. It turns out I suddenly find myself needing to know the plural of apocalypse.
“Dwarves” was coined by J.R.R. Tolkien, due to his fondness for irregular word forms. In a letter to his publisher he called it “just a piece of private bad grammer”. It’s been adopted by later fantasy writers from Tolkien.
In the same letter Tolkien also said “the real ‘historical’ plural of dwarf is dwarrows, anyway: rather a nice word, but a bit too archaic. Still I rather wish I had used the word dwarrow.”
Although, really, almost every other English word ending in F converts it to a V for a plural, so if anything, “dwarfs” is the irregular form. Scarves, knives, lives, staves, so why not dwarves?
Come to think of it, there’s another one with different plurals in different contexts: Long hiking sticks are staves, but it’d be the staffs of the different departments.
Witness the Disney classic, Snow White and the Seven Dwarfs–released the same year as The Hobbit, and thus too early to be influenced by it.
Heh…does thread owe its origin to the postcard thread where I mentioned that line in the card you sent me?
Does he claim he coined it? Not in that letter (to Stanley Unwin, 15 October 1937):
In fact, the OED has this lone cite in its entry for “dwarf”: “1818 W. Taylor in Monthly Mag. 46 26 The history of Laurin, king of the dwarves.”
The counter-exception to this appears to be that the plural is regular for words that end it a double F. Several human corpses are, in slang, stiffs. Several enthusiasts are buffs.
There’s “roof”, but Tolkien also mentioned that he liked “rooves” for the plural.
“Apocalypses” is considered correct, though I’ve never actually seen the plural form used.
Some words and phrases are technically correct but avoided because they sound awkward. I would never say, “There are various apocalypses that are possible and have been discussed.” I would always say instead, “There are many different apocalyptic events that have been discussed because they could in fact occur.”
I’ve never had the need to discuss more than one apocalypse at a time. 
If you search Google Books, you can find plenty of plural usages of the word, often using its primary sense, which is that of a revelation, prophecy, vision.
Nice catch. 
An odd bit about this: many of the times the English plural of circus is used, it’s part of the “bread and circuses” phrase. In Latin, the word used for circus games/events was different than the word for the physical circus ring that they were held in, an adjective used substantively, circensis, with the -ensis ending being the source of the -ese ending in English. Thus in Latin one speaks of “panem et circenses” (accusative plural, since that’s the form that the famous quote in which it derives from uses it) and should possibly be better translated as “bread and circus events”, although English has no problem using “circuses” to mean “circus events”.
One can only speculate on the what-ives of language.
Or anyone. What’s the singular of zucchini? (Hint: Trick question)
Similar things happen in other languages borrowing from a language with different pluralization patterns. In Arabic the words ‘ālim ‘scholar’ and *tājir *‘merchant’ have the broken plurals ‘ulamā’ and tujjār, respectively. But in Persian they often use these plural forms as singulars. Similar to alumni, speakers not recognizing a foreign plural form as plural.
Obviously a trick, since there’s never any such thing as just one of those.