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To cite the duden page for the word Klitoris::
die Klitoris; Genitiv: der Klitoris, Plural: die Klitoris und Klitorides [kliˈtoːrideːs]
I have two questions for that:
If you asked 100 people in the streets for the plural of Klitoris, you’d see a lot of shrugs and hear many formally incorrect guesses based on the common morphologic rules, often probably native *Klitorisse and sometimes perhaps faux-foreign *Klitores or *Klitori. Some will (correctly) assume the null pliural Klitoris and some of them will stress and lengthen the final syllable /…'ri:s/. You’d need to coincidentally meet a gynæcologist or philologist to actually hear Klitorides based on the Greek original.
Some smartass would probably suggest Kitzler – the Germanic (masculine) alternative where null plural is correct for sure. By the way, it’s complicated in English as well: clitoris → clitoris, clitores, clitorises, clitorisses, clitori? It’s not surprising that the shortened but regular clit → clits is rather popular.
Feminine nouns that end in -is, -nis or -ness often refer to abstract concepts in German (e.g. Hybris, Finsternis, Fitness) which almost always occur in singular only. Null plural or E plural are the natural choices. Germanic plural inflection stems are always longer than their singular sibling, i.e. -s < -ds or -x < -ks does not occur. Many educated people know some Latin/Romance and Greek morphologic patterns as well, but it’s not always obvious which one applies, so they are likely to over-correct – even *Klitora, *Klitorae, *Klitoria or *Klitoren (like Basis) may come up, and almost correct *Klitoriden.
To answer the actual question: formal and scientific German usually retains the original plural of loan words, colloquial German tends to find a more regular, more natural alternative, but there may still be various alternarives to choose from and it’s not obvious to everybody which one became convention.
Since I do not know any Greek, ancient or modern, I have to guess, but I assume that Klitorides is the correct plural in ancient or medieval Greek. Now you note that this is not like any plural form that you know, and there you also have the reason why this form is rare in German.
(Not really answering your question, but too long for a comment)
Words with multiple plural forms are not rare in German. It's true, the majority of all nouns has exactly one plural form, but many words have two or even three forms. Also nouns without any plural form exist.
There are lots of words with two different plural form:
Some words also have two plural forms with mean different things:
There are even words with three different plural forms:
And finally there are also words that have no plural form (i.e. they exist only in singular form; there is no way to use them in plural):
btw: Wiktionary knows only one plural form of Klitoris:
(But my spelling checker doesn't know this form, it seems to know only »Klitoris«)
Klitorides is the Greek plural. The word would have been introduced by doctors, who would, at one time, have know Greek, and they would certainly have wanted to show off. Further, medical literature would have been in Latin, where the Greek plural would definitely have been used for a Greek word, especially as the rules for making plurals are not very different in Latin and Greek.
It would also have been a "taboo replacement" i.e. they used Greek to avoid Kitzler which would be considered coarse. Further, professional males managed to convince themselves that hearing such words, in a form they understood, would cause psychological damage to everyone else - the uneducated and females, who did not speak Latin or Greek. Ironically, this had lead to the Latin or Greek words becoming standard for this sort of thing - so everyone knows them now! It is also interesting to note that when German got rid of lots of foreign words in the nineteenth century, such as Oxygen -> Sauerstoff, they failed to get rid of these medical terms.
As for working out what the correct plural is in Greek or Latin, there are three problems for words like this.
clitoris -> clitorides (G)
glans -> glandes (L)
penis -> penes (L) (caught you out - I said usually)
crus -> crura (L) (leg) (plural in -a as neuter)
vulnus -> vulnera (L) (wound)
sperma -> spermata (G) (seed)
About the only clue you might get is if you know a related adjective in English, which usually contains the stem. So penile might help you guess penes and glandular might help you guess glandes. I cannot find any of these adjectives in German. And not many people know the word clitoridal. It does exist - honest! Wiktionary
You might mistake one pattern for another. If you thought clitoris was like penis you would guess clitores which is why you sometimes see this. It is very common to mistake a Greek word for a Latin one like this as more people know Latin.
Yet another possibility is to mistake one of these words for another declension all together. If you thought crus was like cactus -> cacti you would guess cri. This is why people often say octopi, when in fact it is Greek, not Latin, and the plural is octopodes. You might think sperma was like vulva -> vulvae (L) (from which we get the standard modern Italian feminine plural pizza -> pizze) and get spermae.
(Note that I have used c rather than k in these examples as that is the correct Latin, even for Greek words that contain κ. This was the practice in German too. I'm not sure when it changed)
