More than one child? That’s “children”. Add another ox and you have “oxen”. But a “chicken” is just a single one. What’s going on, besides another run-of-the-mill bit of inconsistency in the world’s most inconsistent language?
First the use of “-en” to pluralize a word. That goes back to Old English, where “-en” was commonly used to make some nouns plural. It had to do with some sort of pattern the noun might have; they used “-s” for plural as well. There were any number of words pluralized that way in Old English; eye (eyen), ear (earen), tongue (tongan), house (housen), shoe (shoen), and so on. This practice continued in the Middle English era (Middle English predominated from about 1100 to about 1400), and the use of “-en” for pluralizing was even applied to words to which it was never applied in Old English. Middle English introduced, for example, “devlen” for devil and englen for angel; in Old English those were “deoflas” and “englas.” At the time some there were some other “irregular plurals” as well — that is, words that were pluralized not by “-en” or “-s” but by some other suffix. The plural of “brother” was “brethre,” and the plural of “child” was “childer.” But the Middle English “-en” mob came along and those became “brethren” and “children” — neatly incorporating both the irregular plural and the “-en” version. After a few centuries “brothers” came into use as well (that was in the 1600s), but “children” has just stayed that way ever since.
The only authentic Old English plural using “-en” still in use today is “oxen.” Although “men” and “women” sound like they might qualify (both words come from Old English), they weren’t formed the same way; the “-en” is just a coincidence because those words were pluralized by changing the vowel sound — “man” to “men,” for example, which was also used for “foot” and “feet,” “goose” and “geese,” and so forth.
But back to the chicken. It turns out that the “-en” suffix was used in a couple of other ways besides just for pluralization. In some cases it was added to a word to indicate how something was made or what it was made of. That’s where “wooden” and “woolen” came from). That usage came straight out of Old English, and it’s shared among other Germanic languages.
But wait, there’s more! “Chicken” is singular because Yet Another Way of using the “-en” suffix was as a diminutive. The earliest meaning of “chicken” was “the young of the domestic fowl”, which was, depending on gender, called a hen or a cock. “Chicken” wasn’t exactly formed by adding “-en” to “chick,” either — “chicken” is a very old word, dating back as far as 700, but “chick” didn’t appear until 500 years later, and was originally just an abbreviation. The real source of “chicken” seems to be a prehistoric “Proto-Germanic” word “kiukinan”, which the “Chambers Dictionary of Etymology” speculates was intended to sound like the clucking of…a chicken!
The original sense of “chicken” as a young bird is still echoed in the modern phrase “spring chicken” — it’s almost always used in the negative, as in “he’s no spring chicken.” If you go back to the 1700s, when they presumably had a better recollection of the original meanings of words because they were a bit closer to them (okay, they probably didn’t), the phrase was “he’s no chicken” — they left out the “spring” because after all, “chicken” already meant “young.” And speaking of “spring chicken”, that phrase showed up in the late 1800s, and was pretty specific — it not only meant a small or young chicken, but one precisely between 11 and 14 weeks old. Back in the 1800s they may have still understood that “he’s no chicken” meant no longer young, because the original usage of “spring chicken” was, well, for chickens. What we would call a chicken today could, at the time, have been no chicken at all.
