Size and Weight:
About the Giant Beaver (Castoroides):
It sounds like the punchline to a prehistoric joke: an eight-foot-long, 200-pound beaver with six-inch-long incisors, a narrow tail, and long, shaggy hair. But Castoroides, also known as the Giant Beaver, really existed, and it fit right in with the other plus-sized megafauna of its late Pliocene and Pleistocene ecosystem. Like modern beavers, the Giant Beaver probably led a partially aquatic lifestyle--especially since it was too big and bulky to move about sleekly on land, where it would have made a tasty meal for a hungry Smilodon. (By the way, other than both being mammals, the Giant Beaver was completely unrelated to the beaver-like Castorocauda, which lived during the late Jurassic period.)
The question everyone asks is: did the Giant Beaver build equally giant dams? Sadly, if it did, no evidence of these gigantic construction projects has been preserved into modern times, though some enthusiasts point to a four-foot-high dam in Ohio (which may well have been made by another animal, or be a natural formation). Like the other mammalian megafauna of the last Ice Age, the extinction of the Giant Beaver was hastened by the early human settlers of North America, who may have valued this shaggy beast for its fur as well as its meat.