Josephoartigasia

Josephoartigasia is an extinct genus of giant rodent from the Pliocene. It is related to the living pacarana. The genus includes the largest known rodent, "Josephoartigasia monesi". 
The two species would have looked like gigantic pacarana, or capybaras the size of cows. However, Dr Virginie Millien, from McGill University in Montreal, said the mathematical models originally used to calculate the rodent's mass from its skull probably overestimated its body size.
A recent report said it had big front teeth, which it might have used for defence.