I’m not a huge fan of the official description for purple though. “Very different from their singular forms”? Oxen is literally just “ox” plus “en”. It’s not modifying the singular form at all, just adding something to the end, the same way many words add “s” or “es”. Even “dice” is kinda just “die” plus “s”, with a twist.

I thought of purple as just “plural words made without adding an ‘s’ to the end”.