I shall look at platypus, octopus, cactus, hippopotamus and Pegasus.
The etymologies of the first four.
Platypus is from ‘platipis’, rooted in ‘platus’ (flat) and ‘pous’ (foot). Octopus is from ‘oktopous’, meaning ‘eight foot’. Cactus comes from ‘kaktos’, the name for a prickly plant on Sicily, now known as cardoon.
Hippopotamus is from ‘hippotamos’ meaning ‘river horse’ and ‘hippos ho potamois’ meaning ‘horse of the river’. Both are rooted in ‘hippos’ for horse and potamos for river/running water.
Pegasus has a more lengthy etymology.
Let’s consider the creature first. It is the name for a horse with wings, though nowadays it is being used as the species name for winged horses in general. Horses were creatures of the water, hence in the realm of Poseidon, who is also Pegasus’ sire. It also had wings, meaning it was also a creature of the sky, meaning that it also belonged in the realm of Zeus. Indeed, Zeus used Pegasus to carry his lightning bolts.
It could be from ‘pege’, meaning ‘fountain/spring/well’; it could be from ‘pegos’, meaning ‘strong’. Another possibility is from the Luwain ‘pihassas’, meaning lighting’, as from their weather deity Pihassassi. As the Greeks were a maritime people, this meant that they would have had contact with many other cultures, including the Luwain, even if it were only indirectly through another culture. It is likely that all of these influenced the development of ‘pegasus’ as a word.
With any combination, linguistic reductionism would have had to occurred. Linguistic reductionism is when a word is gradually reduced when spoken, because speech is for communication, and the faster one’s idea is passed on, the easier it is to communicate. Keeping linguistic reductionism in mind, the three possible combinations of two words all seem highly plausible: ‘pegepegos’, ‘pegospihassass’ and ‘pegepihassas’.
As with words ending in ‘-us’, people think that the ‘proper plural’ should be ‘-i’. However, this is only if the words has come from Latin. If a word is from Greek, the plural for ‘-us’ is ‘-odes’. The above five are from Ancient Greek.
Now, this causes all sorts of drama. Does one follow the rules from the language that the word originates from, or the rules of the language that it travelled through to us? Or, as with most words that come from a foreign language, should they eventually adopt the plural rules of the host language?
This, of course, is following the Prescriptive view of spoken language, giving it rules for use and ‘correct’ punctuation. Language is always developing, otherwise how would there be thousands of different languages? If language didn’t change, we’d all be speaking the same language. If change in language ‘shouldn’t be done’, then we should go back to the primordial language(s) of humanity… From what we have available, this would be impossible to do.
I’m not saying we shouldn’t have rules at all. Obviously spoken language does have rules, but the majority aren’t imposed from sources other than the language’s evolution. Written language definitely should have rules because it is used in a formal capacity, and with anything formal, there are rules and regulations to go with it.
Now, as plurals are originate in spoken rather than written language, plurals don’t follow one set ‘should’ rule. This would mean that the five above words could end in the Greek plural ‘-odes’, the Latin plural ‘-i’ and the English ‘-es’. Really, what’s wrong with that?
I will demonstrate by having the singular, then the Greek plural ‘-odes’, the Latin plural ‘-i’ and the English plural ‘-es’.
Platypus. Platypodes. Platypi. Platypuses.
Octopus. Octopodes. Octopi. Octopuses.
Cactus. Cactodes. Cacti. Cactuses.
Hippopotamus. Hippopotamodes. Hippopotami. Hippopotamuses.
Pegasus. Pegasodes. Pegasi. Pegasuses.