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.