The biggest rodent in the world still living today is the capybara(Hydrochoerus hydrochaeris). Capybara are semiaquatic mammals that live throughout South America. Their closest living relatives include chinchillas and guinea pigs, but unlike those rodents, Capybara can clock in at over 4 feet in length and 150 lbs! They are very social animals, and tend to live in groups ranging from 10-20 individuals.