I grew up on a farm with cows. A lot is going to depend on the breed, and the personalities of particular cows. High strung breeds can be the most dangerous...especially if they have horns. Cows will tend to move away from strangers unless there is a bull on alert or an ornery cow...especially if that cow has a calf. I've not seen horned cows get aggressive but I've seen polled angus get seriously riled. After they knock you down they butt hard at your head and body while on the ground. We had one coward cow...until she had a calf. Got my uncle down and broke four of his ribs. He had to roll under a fence to get away. If you intend to steer a running herd, best have a cow dog and a quarter horse and know how to use both. Trampling could be a problem in close quarters. Horses will try to avoid stepping on you if they can (would rather bite and kick), but not cows...if you are in the way, you're a welcome mat. It should be noted, unlike horses, a cow cannot kick straight back. Safest place in a bunch of panicked cows...close behind the slowest one. They can kick to the side and back. Another not recommended escape plan involves spooking the cows into a run, grabbing on to a tail, sitting down, and butt surfing whichever way they are headed. Hope for no weak vertebrae and no cactus, and a cow too invested in getting away to stop and look at what's behind her. We had an English shepherd that used to do this...sort of funny to watch.
If you are in a pen with milk stock or work stock (like oxen) you have the opposite problem. They like people, expect to be fed and may knock you down if you don't. This is especially true of the bulls. Farmers make pets of them so they can handle them to medicate them and such. Some of them become very spoiled pets who don't (or perhaps do) know their own strength relative to a human...these are harder to drive and need to be coaxed and persuaded (cow dogs can help with the persuading). However, if you have feed, they will follow (may trample you a little to get to it)...but if you are dropping sweetened grain you can hide by hunching down and just walking away. To a distant observer, just a herd of cows headed to pasture. Don't try riding one. It scares them and they just spin and buck in a circle....you won't get anywhere but on the ground. Their backs are also a ridge of bone...except on very fat one...very uncomfortable to sit astride. If you want real shock interest, then write you a herd of these into the story...double muscled cows:
http://i.ytimg.com/vi/WseH479iOM0/maxresdefault.jpg
Addendum...braining a cow might not be as effective as you might imagine. Their heads are pretty hard and they butt other 1500 lbs animals head to head when they fight. A ball pean may concentrate force better than a maul (which will hurt them...but nothing close to a guarantee)... The way to 'safely' pacify anything but an utterly enraged cow (and maybe even then) is your thumb and forefingers....reach in their nose and pinch their septum hard...very sensitive...that is the point of the rings in bull noses. There are also special round tipped pliers for this in you can find them in time. You can also try grabbing the ears and twisting the head over hard...works best on juveniles and young cows. It causes them to loose their balance and fall over...if you are macho enough and strong enough you can generally do this with a cow (she wont be happy). Don't try this with a bull, you'll just irritate it. Nose pinch is better (pinch and pull upwards...cow will raise head and open mouth). Aka...what works with cows, does not work with Bison...can be lead, but almost impossible to herd.