Rats!

Or, a better mousetrap ain't gonna do it. From the inevitable PZ Myers:

Now there's another king of the rodents: Josephoartigasia monesi, which is estimated to have tipped the scales at about 1000kg, over a ton. Don't worry about getting bigger rat traps; these beasties have been extinct for perhaps 2 million years.

See Pharyngula for a picture. Charming brute, isn't it?