|Clearly, This Guy is not a Real Viking|
There are a lot of misconceptions about Vikings. People think they were a bunch of blood-thirsty marauders who wore helmets with horns and went around raping and pillaging. The time has come to put a stop to this nonsense. The Vikings did not wear helmets with horns. If you think Vikings had helmets with horns, you're just living in a fool's paradise.
When it came to the other stuff, the Vikings were all over it: raping wives and daughters, burning crops, and carrying off livestock - and any other permutation of the preceding nouns and verbs you care to name. One thing the Vikings loved was a good monastery. A monastery was like ice cream to Vikings, especially if it had priceless works of religious art and stuff.
Rape and pillage is all well and good, but sooner or later, you want a regular job. The Vikings eventually decided instead of raping other men's wives and daughters and stealing their crops and livestock, they'd just move in, have their own wives and daughters, crops and livestock, and thereby cut out the middleman. After this, they stopped being Vikings and began being Danes.
When the Danes started moving in, it was a good thing and a bad thing. It was a bad thing if you happened to occupy the real estate they had an eye on, but it was a good thing because it was marginally better being neighbor to a Dane than an out-an-out Viking. With a Dane, you could hope to reason with him. You could say more than, "Take my wife, please, but don't stab me with that... arggghhh." The catch is, if you were going to talk with a Vik- excuse me, Dane, you had to be able to talk.
What the Danes spoke was Old Norse, only, naturally, they didn't call it that, and I don't know what they did call it, so don't ask. The catch was, Old Norse was very, very similar to Old English, but not exactly. Think of a Mexican talking to a Brazilian, something like that. You can make yourself understood provided you aren't too persnickety about grammar.
The heading on this blog promises to answer why we say "mice" instead of "mouses" and "geese" instead of "gooses." A better question is why there aren't more even more wierd-o plurals. Why don't mothers - excuse me, mothern - say, "Children put on your shoen before you ride the oxen, or you might poke out your eyen." The fact is, that's exactly what an Anglo-Saxon might have said, in an admittedly bizarre set of circumstances.
Old English had several sets of words, each with its own rules for pluralizing. Some, you just added a good old -s to the end, like microwave ovens and emoticons, and some you changed the vowel sound of, like men or mice and geese, and some you added an -n to, like oxen and children and brethren. But it used to be, there were a lot of words in each of these groups, but after the Danes, not so much. If your Danish neighbor asked you, "How much do you want for those horses of yours?" you wouldn't say, "That's horsen, you dolt, you're using the wrong declension," unless you wanted to see him forget his manners and go all Viking on your ass.
At the end of the day, the grammar of Old English simplified and became less synthetic and more analytic. In short, it mattered less what form a word took, than its position in the sentence. This process of simplification when one language comes in contact with a related, but different, one, is called rubbing. And when it comes to rubbing, no one did it better than the Danes, with or without horned helmets.