The Giant Beaver (Castoroides ohioensis) wins the prize for the largest rodent, at least in North America.    From Wordnik.com.  [Giant Beaver! The Bigger They Are: 10 Ice Age Giants | Disinformation] Reference

The Giant Beaver (Castoroides ohioensis) was a huge species of rodent, with a length up to 2.5 m (8.2 feet) and an estimated weight of 220 kg (485 lbs).    From Wordnik.com.  [Dr. West, meet Dr. Tinkle, Creationist eugenicist - The Panda's Thumb] Reference

(Canis dirus); sabertooth cat (Smilodon fatalis); American Scimitar cat (Homotherium serum); giant beaver (Castoroides ohioensis); beautiful armadillo (Dasypus bellus); American camel.    From Wordnik.com.  [ScrippsNews] Reference

Castoroides ohioensis.    From Wordnik.com.  [Dr. West, meet Dr. Tinkle, Creationist eugenicist - The Panda's Thumb] Reference