Casroroides ohioensis was a giant beaver that lived a few thousand years ago. It is generally believed to be extinct, but people do report sightings of giant beavers today.
A few months ago, I read John Warm's book Strange Creatures Seldom Seen, which came out in April of this year. In it, there is a whole (long) chapter on sightings of giant beavers.
Warms recounts lots of sightings of the creatures, including his sighting of what may have been one. He also discusses (and has pictures of) tunnels that are thought to have been made by the beavers. And those tunnels are gigantic. There are also pictures of giant beaver dams. (Which are also gigantic.)
Native Americans told stories of these huge beavers (just like they did with Sasquatch and Thunderbirds) and they have been reported from various places in Canada and the United States. For example, the Mi'kmaq people of Canada and the Pocumtuck people of Massachusetts both have legends of giant beavers.
Lots more information on sightings of giant beavers can be found in Warms' book, as well as in Loren Coleman and Patrick Huyghe's Field Guide to Lake Monsters, Sea Serpents, and Other Mystery Denizens of the Deep.
Do you think the giant beaver could still be alive?