randomreindeer replied to your post “today i learned why some words like deer don’t have a plural and i...”
essentially it has to do with declensions in old english. OE used to have inflections for nouns like we have for verbs so like how we say “I run” but “she runs” they would do the same thing with nouns based on the number and the case (nominative, accusative, etc). There was grammatical gender too like french but that’s a whole other thing.
the way different words got inflected for singular vs plural depended on their root/stem. a-stem words were the most common and the masculine a-stem form became a plural by adding an -as. Over time this became -as, -es, and -s and most words followed this through analogy (because language is complicated the most common things get extended to almost every situation to make it easier on our brains).
Words like deer, however, came from the long neuter a-stem that had an endingless plural
oxen was a weak noun and is, fun fact, the only weak noun to survive with that form of plural ending (children is almost but also has the r plural ending from a different form)