How Can I Butcher Animals en Masse?

So, my friend just implemented a bunch of new mods into his SMP server, which meant a host of new commands to play with.

I thought I'd give the /mobspawn command a few goes, and see what I could achieve. So I headed off to the server's jail (I forget why this particular location compelled me), and gave it a few spins.

I seem to have made a bit of an error in judgement.

What can I do to get rid of this mess? Is there a command or some other technique I can use to clear out any mobs in my immediate vicinity?

Depending on the mods installed, /butcher [radius] should work. Of course, you'd need access to the commands to do so.

This is now possible in vanilla with a feature introduced in the 1.8 snapshots. You can now /kill animals and mobs using the @e selector. For instance to kill all the pigs in that pen, you would use

/kill @e[type=Pig,distance=..10]

And similarly, you'd use Cow instead of Pig to kill all the cows in the pen. I use the radius specifier so that you don't kill all the pigs or cows that are currently loaded in your world, just the ones that are within your vicinity (10m). Note that there's only one space, between /kill and @e, in this command. Adding additional spaces will cause the command to not parse properly, and will not only kill you, but possibly every other entity that's loaded in the world.