This only goes so far back. I believe they actually came from German. Oxen in German is Ochsen German still actively uses the -en plural, and all nouns in the dative plural must use -n.
So, Ochsen -> Oxen, Maenner -> Men
(Slightly unrelated) So if you look at Scotland and Ireland, they've get some really interesting things in their dialects. die "Kirche" in German is a "church", in some dialects in scotland, it's a "kirker". Or in old dialects in Ireland, "children" is sometimes spoken as "childer".
EDIT: I retract my theory on the relationship between children and kindern :)
So, Ochsen -> Oxen, Maenner -> Men
(Slightly unrelated) So if you look at Scotland and Ireland, they've get some really interesting things in their dialects. die "Kirche" in German is a "church", in some dialects in scotland, it's a "kirker". Or in old dialects in Ireland, "children" is sometimes spoken as "childer".
EDIT: I retract my theory on the relationship between children and kindern :)