The capybara is the current champion for rodents of unusual size — it weighs about 60kg (about 130 pounds); another large rodent is the pakarana, which weighs about a quarter of that. Either one is far too much rattiness for most people to want hanging around.
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. I’ve put a few pictures from the paper describing this new species below the fold.
This is the skull of the type specimen.
To help get an idea of the size of this skull, here’s a pakarana skull (left), capybara (middle), and J. monesi (right). It’s huge!
And what did this rodent the size of a cow look like while alive? Here’s a reconstruction, drawn to scale with a pakarana.
There is no suggestion that it lived in a fire swamp — its habitat was probably forested river deltas.
Rinderknecht A, Blanco RE (2008) The largest fossil rodent. Proc. R. Soc. B doi:10.1098/rspb.2007.1645.