It's the mightiest mouse the world has ever seen - one of the really big cheeses of the animal kingdom.
The prehistoric throwback - easily capable of putting the cat out - was bigger than a bull and weighed around a TON.
It lived four million years ago in the South American rain forest. The super-rodent's existence was revealed after a complete fossilised skull was found in Uruguay.
Scientists say it comes from the Dinomyidae family of mice - but is far bigger than anything discovered so far.
Writing in the Royal Society Journal of Proceedings B, they reckon it could have grown to 2,500kg.