That’s not true for at least 3 of the OP’s examples - brethren, children, and oxen, all of which are proper Germanic plurals.
I found a pretty good web page listing irregular English plurals. Some words from that page not already listed here:
calf - calves
elf - elves
half - halves
hoof - hooves
life - lives
loaf - loaves
scarf - scarves
self - selves
shelf - shelves
wife - wives
wolf - wolves
analysis - analyses
axis - axes
basis - bases
crisis - crises
diagnosis - diagnoses
emphasis - emphases
hypothesis - hypotheses
neurosis - neuroses
oasis - oases
parenthesis - parentheses
synposis - synposes
thesis - theses
“Children” is a double plural. “Childer” (cf. German Kinder) was the historic plural, but since even early English didn’t pluralize with -R much, then -EN plural was added. Dialectal “chilluns” is a triple plural (although plural #1, the -R, has vanished away).
But in fact all of the OP’s examples are regular plurals. Pluralizing by umlaut (raising the internal vowel) is a perfectly fine is uncommon way of forming plurals. Compare Welsh *asgwrn * (bone) / *esgyrn * (bones). And there is no mention of words like person, whose plural is either the regular *persons * or the highly irregular people, taken from a different root altogether. I think what the OP is asking is “uncommon English plurals derived from Germanic roots.”
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So we have the vast majority of nouns pluralizing by -s / -es.
Some of these make the list because of subsequent phonetic changes in English: F and V were originally allophones; so word-final -F was pronounced /f/ (cnif > knife) and in between two vowels it was pronounced /v/ (cnifes > knives). Also die > dice. -
A few remanants of earlier ways of forming the plural: n-stems by adding -n / -en instead of -s / -es (ox, oxen) or athematic consonant stems by umlaut (mouse, mice)
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children, an old -r / -er plural “regularized” to and -n / -en plural.
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plurals taken from other cases and used in special circumstances (penny, pennies / pence).
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nouns where a different word altogether is used to make the plural (person, people).
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Latin plurals: 1st declension -a > -ae (alumna), 2nd declension -us > -i / -um > -a (datum, data), third declension -is / -es (crisis, crises)
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Greek plurals: -on > -a (phenomenon, -a), plus some other ones (my Greek isn’t good enough for a list).
Have just looked at Roger Lass’s Old English. He says:
“In post-OE times, only foot, goose, man, louse, *mouse * have not gone over to” adding a plural ending, but once the group included the words] oak, burgh, cow, goat, turf, furrow, nut. “The plural of brôc, i.e. brêc, does remain in the double-plural form breeches, though the singular (which would be *brook) has been lost.” (p. 136)
He also notes that calf, lamb, and egg all used to have -r plurals. (OE: cealfru, “calves”)
You could argue that these are regular, because English has a rule that nouns endding in -f or -fe form their plural with -ves. However, if that is the rule, then the dwarfs form of the plural of dwarf becomes irregular!
genie - genii
Biceps is singular, and the plural is bicepses, according to my old 12 pound Webster.
Kudos is singular. It seems to be one to a customer, because there’s no plural listed.
One more irregular plural: “Maple Leafs” (as in the Toronto hockey team). If “leaves” is the regular plural of “leaf”, then this is irregular. (They really do need to define what a “regular plural” is.)
I would assume it has a plural in Greek.
I once read a columnist decrying the fact that people pronounce it “koo-doze” as if it were plural, instead of “koo-dos”, and that some people acually refer to a “kudo” as a single act of praise. I think he said something like-- what’s next, a single instance of feeling a patho?" It seemed funny at the time.
A good book for those interested in regularity and irregularity in language is Words and Rules by Steven Pinker. Much of the book is about irregular verbs and nouns: where irregularity comes from, how it persists, how people learn them and what this says about how the brain processes language.
And there is a good Wikipedia article on English plural, asccording to which many other words have irregular plurals.
It’s been many years since HS German, but isn’t “kindern” also a perfectly acceptable dative plural? If so it seems that “children” has simply been taken from the dative and so follows regular Germanic pluralisation.
I’m not so sure on this one: if someone referred to their ‘brethren’, I’d assume that they were not biologically related.
Interesting semantic drift. The word “brethren” is now most commonly used in certain religious organizations to refer to members of the group. The singular is still “brother,” which is how a member would refer to a single male member of the group. One might also hear the word “sistern” in this context to refer to female members of the group. Anyway, you are able to deduce that “brethren” doesn’t refer to blood relatives because the word is now used only in contexts where it means “metaphorical brothers.”
They also left off mongoose -> polygoose.
Polygeese, silly.
Not only is it still current, it actually seems to be gaining ground over the singular. “One pence” is heard quite commonly – in fact Orange (the mobile phone company) had big billboards up a couple of years ago advertising text messages for “ONE PENCE EACH”. Ugh.
Any Slvester fan can tell you that the plural of “mouse” is “meeces”.
Index/indices?
“Chickens” is as well, since “chicken” used to be plural (singular “chick”, of course).