There's a theory that, at one time, "man" was a gender neutral term for persons and we called males "weremen" and obviously women "women"

That probably isn't true, but it's fun to think about a world where that was reversed.

EDIT Also evidently there was wifmann and wapman, which is an even funnier world to imagine.

Not what you asked for, but Welsh has irregular plurals for adults and children, too:

Adult: oedolyn
Adults: oedolion

Child: plentyn
Children: plant

(Still learning, so I'm not too sure why or how)

I’ve met some sweet southerners that already call children “childs.”