Q: Your recent post about why “chicken” is singular has left me wondering where “-en” plurals such as “oxen,” “brethren,” “children,” “men,” “women,” and the archaic eyen come from.
A: In Old English, nouns that followed certain patterns formed their plurals with -n rather than –s.
These included the one you mention, eyen (“eyes”), as well as earan (“ears”), tungan (“tongues”), fon (“foes”), housen (“houses”), shoen (“shoes”), treen (“trees”), and oxan (the original plural of “ox”).
During the Middle English period (roughly 1100-1400), both the –en and the –an plurals that had come from Old English were spelled with –en.
Meanwhile, Middle English writers extended the -en spelling, applying it to words that didn’t originally have plurals ending in –n.
As the Oxford English Dictionary explains, “the termination -en came to be regarded as a formative of the plural, and its use was extended in southern Middle English to many other words of Old English and French origin.”
Charles Barber, Joan C. Beal, and Philip A. Shaw note in The English Language: A Historical Introduction (2nd ed.) that Middle English had “forms like devlen ‘devils’ and englen ‘angels,’ where Old English had deoflas and englas.”
This -en ending was so popular in Middle English that it was even added to existing irregular plurals, so that brethre (plural of “brother”) became brethren and childer (plural of “child”) became children.
You might say that the –en of “brethren” and “children,” added to words that were already plural, formed in each case a sort of double plural. (The modern “brothers” wasn’t commonly used until the end of the 16th century.)
For a time, –n and –s rivaled each other as the typical plural ending in English, Thomas Pyles and John Algeo write in The Origins and Development of the English Language (4th ed.).
In general, the -n ending was favored in the south of England and the –s in the north.
But nearly all of the –n plurals eventually disappeared as the –s plurals became dominant. By around 1400, say the authors of The English Language, the –s plurals were “almost universal.”
The only original –n plural from Old English that has survived to this day is “oxen.” And even this plural had a run for its money. It competed for a time with “oxes,” which the OED says “has survived only in regional and nonstandard use.”
(The plurals “men” and “women,” by the way, don’t fall into this category. They were formed in Old English by a change of vowel, as is also true of “feet,” “geese,” “teeth,” “mice,” and “lice.”)
We should mention a couple of other points about –en endings in English.
As we wrote in our “chicken” post, the –en suffix has been used to form diminutives. This is the case with the –en of “chicken,” “kitten,” and “maiden.”
And –en has been added to nouns to form adjectives in the sense “pertaining to” or “of the nature of,” the OED says. In Germanic languages, adjectives formed this way “chiefly indicate the material of which a thing is composed,” Oxford adds.
Only a few of these adjectives survive today in English, including “golden,” “wooden,” “leaden,” “oaken,” “woolen,” “earthen,” and “wheaten.”
from Grammarphobia http://www.grammarphobia.com/blog/2016/03/en-plural.html